Eliminate skin updates in between activities

1) Adds way to pop activity without refreshing the skin at
the same time.

Activities are sometimes popped in immediate succession,
or one activity is popped before another one is pushed right
away. This can lead to the UI appearing glitchy, due to an
activity only appearing for a split-second, which is especially
noticeable with complex skins that change the dimensions
of the UI viewport depending on the current activity

To fix this, prevent superfluous skin updates

* when switching between:
- WPS and browser
- WPS and Playlist Catalogue
- WPS and playlist
- WPS and Settings/System/Plugins

* when accessing Track Info or when displaying
bookmarks using the context menu on the WPS

* when switching from QuickScreen to Shortcuts Menu

2) The playlist viewer activity was pushed & popped
redundantly by playlist_view.

----
NB:
Behavior has remained unchanged in all instances of the
code where pop_current_activity() has been replaced by
pop_current_activity(ACTIVITY_REFRESH_NOW).

Change-Id: I56b517b8c9dba823a9fed3a3f558d7469dcea9fd

apps/plugin.c

@@ -919,7 +919,7 @@ int plugin_load(const char* plugin, const void* parameter)
     int rc = p_hdr->entry_point(parameter);
 
     tree_unlock_cache(tree_get_context());
-    pop_current_activity();
+    pop_current_activity(ACTIVITY_REFRESH_NOW);
 
     if (!pfn_tsr_exit)
     {   /* close handle if plugin is no tsr one */