GtkAppChooserWidget: Monitor installed applications

This uses the new GAppInfoMonitor object in GIO, which provides
change notification when the set of installed applications changes.
GtkAppChooser: Drop PackageKit integration

This will come back in a simpler form in the next
commit.
GtkAppChooserDialog: Add a 'Software' button

Add a simple button that launches GNOME Software.
Remove an unneeded include
Update config.h.win32.in for Visual C++ 2013

A number of C99 math.h functions, along with inttypes.h were included for
Visual C++ 2013, along with much improved C99 capabilities, so update
config.h.win32.in to reflect this.
GtkAppChoserWidget: Don't leak the context menu

This was pointed out in
https://bugzilla.gnome.org/show_bug.cgi?id=720401
Fix a compiler warning
GtkColorSelection: Don't leak a context menu

This was pointed out in
https://bugzilla.gnome.org/show_bug.cgi?id=720401