desktop-app-info test: use g_assert_strcmp()

Replace some assert(strcmp()) with g_assert_strcmp() so that we get
better output in case of failures.

https://bugzilla.gnome.org/show_bug.cgi?id=728040
mimeapps test: fix defaults vs. recommended

The desktop file for myapp3 didn't declare support for image/png, but
the testcase expects it to be recommended on the basis of it being the
default app according to defaults.list.

This will not work in the future -- we will only list apps that actually
support the filetype in question, unless they've been explicitly added
as associations.

https://bugzilla.gnome.org/show_bug.cgi?id=728040
tests: expand 'apps' tool

Add modes to output the applications found for get_default_,
get_recommended_, get_fallback_ and get_all_for_type().

https://bugzilla.gnome.org/show_bug.cgi?id=728040
appmonitor test: avoid /usr/share

Set XDG_DATA_DIRS to make sure we don't use /usr/share from the
appmonitor test.  We will soon throw a warning if we find defaults.list,
so make sure we don't open ourselves up to that if there is one on the
system.

https://bugzilla.gnome.org/show_bug.cgi?id=728040
mimetype tests: don't depend on specific behaviour

We currently assume that setting an application as the default will take
it to the front of the list of supported applications for a given type,
but this is not necessarily true.

Check instead that the application is actually set as default.

https://bugzilla.gnome.org/show_bug.cgi?id=728040
GDesktopAppInfo: rewrite content type code

Redo the code for type-based selection of applications (all,
recommended, default, fallback) based on the new DesktopFileDir
structures that we introduced last cycle.

At the same time, we expand the functionality to add support for the new
features of the specification:

 - moving ~/.local/share/applications/mimeapps.list to ~/.config/

 - per-desktop default applications (via XDG_CURRENT_DESKTOP)

 - sysadmin customisation of defaults (via /etc/xdg/mimeapps.list)

 - deprecation of the old defaults.list, favouring the use of
   /usr/share/applications/mimeapps.list (or gnome-mimeapps.list) to
   accomplish the same

We modify the mimeapps testcase to check for mimeapps.list having been
created in XDG_CONFIG_HOME instead of XDG_DATA_HOME.

The modification is a net reduction of code (due to less duplication in
bookkeeping).  It is also an increase in performance and reduction in
memory consumption (due to simplified data structures).  Finally, it
removes the stat-based timestamp checking in favour of the
GFileMonitor-based approach that was already being used in the
implementation of DesktopFileDir (in order to know if we had to rescan
the desktop files themselves).

https://bugzilla.gnome.org/show_bug.cgi?id=728040
tests: use mimeapps.list over defaults.list

defaults.list is deprecated, so use mimeapps.list as a filename instead.

https://bugzilla.gnome.org/show_bug.cgi?id=728040