Fix bug 16506: Honor read access given by ACLs.

For this, use "access" instead of stat.
More BMF performance tweaking

In a first pass we're only going to check file/folder paths for
existence. We don't need the actual track object in this case. Thus a
quick DB lookup should be good enough in most cases. Only new items need
to be scanned.
More BMF tweaking

When we first read a folder's content, we only want to know whether an
item should be shown or not. We don't need to create track objects at
this stage yet, as we're going to page through the items later. In the
case of a folder with a lot of items in it we would have created a track
object for every single item, though we probably only need the first
fifty (web UI) or 200 (player UI) items. 

Speed up initial menu creation by only checking for the content type,
without creating the track objects in the database. Only create them as
we're paging through the full result list. This should return control to
the player much quicker, with less risk of interrupting playback, timing
out etc.
Enforce extensions downloader plugin. Too many users disable it
accidentally, later complaining about the lack of plugins.
Merge branch 'public/7.8' into public/7.9
Improve web UI browsing performance by caching rendered HTML pages.
These pages should not change between scans of the library.
Persist "New Music" album IDs across server restarts.
Fix cache key for rendered web pages
Only cache select browse modes. Make sure we don't mix up languages,
sort orders, skins etc..
Bump lastScanTime when using BMF. We don't want to miss data updates
caused by browsing folders.
Fix browsing New Music cache
Don't cache XMLBrowser action items in the web UI
Fix support for multi-disc sets in cue files

Allows both DISC and DISCNUMBER to be used for disc number.

Allows DISCC, DISCTOTAL, or TOTALDISCS to be used for total
number of discs in the set.

Permits quotes to be optional as disc numbers might not have them.

Reference:

http://bugs.slimdevices.com/show_bug.cgi?id=17770
Make sure we respect album/artist/genre/year IDs when drilling down our
local music collection in the web UI.
Make main web menu snappier. Don't rely on JS to expand/collapse items
upon page load.
Merge remote-tracking branch 'mrthreeplates/fixcuedisc' into public/7.9
Changelog update
Merge remote-tracking branch 'htgoebel/public/7.8' into public/7.9

Change-Id: I580031ce8970729a3467b06bcbd999b02836e56d
Windows doesn't like "use filetest 'access'". Disable it.

Change-Id: I74b08067af01fde1e53f7761c6d9f95e98a3ccb0
Bug 18089: "View Tags" for the file without any tags breaks the web UI.
Make sure we return correctly if we're lacking a callback.