skin: add %pX tag for time-based playlist progress

%pP reports playlist progress by position index, which treats every
track as equally long. For playlists with tracks of unequal length --
audiobooks with chapters anywhere from two minutes to an hour are the
motivating case -- position is a poor proxy for listening progress.

%pX reports the played percentage of the whole playlist by time: the
summed length of all preceding tracks plus the elapsed time in the
current one, relative to the playlist's total duration. It can be
used as a value, in a conditional, with %if(), or as a bar tag like
%pb.

If a playlist contains more than 500 tracks or the scan is taking too
long and the user aborts the tag will fallback to the behavior of %pP
except the progress through the current track will be included in the
returned percentage

--------------------------------------
Computing this needs every track's length, and reading metadata for
every track is too slow and disk-heavy for a tag that refreshes on the
WPS. Instead each track's length is estimated from its file size: the
skin engine scans the playlist in the background, a batch of files each
skin refresh, opening each file only to read its size (directory
metadata, no header parse). Size is turned into time by calibrating one
file of each type -- the first file of each extension is parsed once
with get_metadata to learn its bytes-per-second, and every later file
of that type reuses it. A single-format playlist, the usual audiobook
case, parses exactly one file and stat's the rest.

The result is an estimate -- bitrate varies within a type, especially
for VBR -- but it is cheap and accurate enough for a progress
indicator, and the playing track always contributes its exact elapsed
time. Per-track lengths are stored as two bytes of minutes each in a
movable buffer sized to the track count and allocated only while the
tag is in use; the cache is keyed on the track count and a crc of the
first, middle and last filenames, so a playlist swap or reshuffle is
caught. If the buffer cannot be allocated (a very large playlist on a
low-memory target) the tag falls back to position-based progress.

The buffer is allocated when playback starts (and when the now-playing
screen is opened with playback already active) rather than lazily on the
first WPS refresh, so the one-time allocation happens at a playback
boundary instead of during steady-state playback; it falls back to
allocating on first use if that point is missed.

Until the scan finishes the tag does not blank: it returns an instant
equal-weight estimate -- (completed tracks + fraction through the
current one) / track count -- which sharpens into the size-based value
as the scan fills in. So a theme can just use %pX and it is correct
from the first frame, and %?pX is true whenever a playlist is loaded.

Tested on a Sansa Clip Zip and in the simulator. On a 216-track,
~745 MB single-format audiobook playlist the length scan dropped from
~4150 ms (get_metadata on every track) to ~174 ms (one parse plus 215
file-size stats), roughly 24x lighter.

Change-Id: I6e572e78a10444bd513ddc77e30da04aa5153ef2

manual/appendix/wps_tags.tex

@@ -176,6 +176,15 @@ or \config{\%D(2)}), produce the information for the next file to be played.
     \config{\%pp} & Playlist position as index\\
     \config{\%pP} & Playlist position as percentage. Can be used as a value, %
             a conditional tag or a bar tag.\\
+    \config{\%pX} & Percentage of the whole playlist played by time. Unlike
+         \config{\%pP} this reflects tracks of differing length when 500 or less, 
+         such as audiobook chapters. Othewise acts like \config{\%pP} except with
+         also including progress through the current track in the percentage.
+    \opt{HAVE_DISK_STORAGE}{
+         Track lengths are estimated from file size to reduce disk use, so
+         the value may be approximate.
+         }
+         Can be used as a value, a conditional tag or a bar tag.\\
     \config{\%pr} & Remaining time in song\\
     \config{\%ps} & ``s'' if shuffle mode is enabled\\
     \config{\%pt} & Total track time\\