gdesktopappinfo: remove should_ping_mime_monitor

This variable is used for nothing...
gdesktopappinfo: remove global_defaults_cache

...and some related functions.

This variable is also used for nothing at all.
gdesktopappinfo: rework search path

Change the search path to be a global array of 'DesktopFileDir' structures and
change the 'get' function to an 'ensure' function.

This is just a straight-up refactor.  Future patches will expand the
DesktopFileDir structure.
gdesktopappinfo: Large-scale whitespace fixup

Fix up a lot of whitespace issues in this file since we're about to do
some pretty serious rewriting here anyway...

Add some fold markers while we're at it.