Database: Support for multiple search roots.

Support multiple roots (i.e. scan folders) using the new folder_select()
function.

Amaximum of 12 dirs can be selected, however the setting (i.e. the entire
folder list) cannot be longer than 80 chars.

The setting works similar to the autoresume dirs: Directories are seperated
by colons, e.g. "/Music:/Podcasts". Default is "/sdcard" on android, "/" on
all other targets.

Changes are made to the multiple-root logic in tagcache.c. This is to
a) provide the logic at all on native targets and b) support more than
one root before the scan even starts (until now it was used to add roots
during scan due to symlinks).

Change-Id: I64992c0678324536e8e64cd4427c8abbd8e8b39e

apps/tagcache.h

@@ -201,8 +201,6 @@ struct tagcache_search {
     int32_t idx_id;      /* Entry number in the master index. */
 };
 
-void tagcache_build(const char *path);
-
 #ifdef __PCTOOL__
 void tagcache_reverse_scan(void);
 #endif