This dramatically thins down and sanitizes the application code.

The ShellAppSystem changes in a number of ways:
* Preferences are special cased more explicitly; they aren't apps,
  they're shortcuts for an app), and we don't have many of them, so
  don't need e.g. the optimizations in ShellAppSystem for searching.
* get_app() changes to lookup_app() and returns null if an app isn't
  found.  The semantics where it tried to find the .desktop file
  if we didn't know about it were just broken; I am pretty sure no
  caller needs this, and if they do we'll fix them.
* ShellAppSystem maintains two indexes on apps (by desktop file id
  and by GMenuTreeEntry), but is no longer in the business of
  dealing with GMenuTree as far as hierarchy and categories go.  That
  is moved up into js/ui/appDisplay.js.  Actually, it flattens both
  apps and settings.

Also, ShellWindowTracker is now the sole reference-owner for
window-backed apps.  We still do the weird "window:0x1234beef" id
for these apps, but a reference is not stored in ShellAppSystem.

The js/ui/appDisplay.js code is rewritten, and sucks a lot less.
Variable names are clearer:

_apps -> _appIcons
_filterApp -> _visibleApps
_filters -> _categoryBox

Similarly for function names.  We no longer call (for every app) a
recursive lookup in GMenuTree to see if it's in a particular section
on every category switch; it's all cached.

NOTE - this intentionally reverts the incremental loading code from
commit 7813c5b9.  It's fast enough
here without that.

https://bugzilla.gnome.org/show_bug.cgi?id=648149