Improve GInitable test coverage

This just adds a bit more variety to the uses of GInitiable
in other tests.
Improve GSubprocess test coverage

Test the synchronous communicate functions.
Improve GApplicationCommandline test coverage

Add tests for many GApplicationCommandline apis to the
existing application tests.
Simplify fileattributematcher tests a bit

Just use assertions instead of pretty-printed error messages.
Add tests for x-content type sniffing

This test exercises the tree matching parts of gcontenttype.c
Trivial formatting fixes
Make GEmblem more robust

No reason not to survive g_object_unref (g_object_new (...)).
Make GFileIcon more robust

No reason not to survive g_object_unref (g_object_new (...))
GTlsPassword: Make declared defaults match

String properties typically start out with a value of NULL.
No reason to deviate from that convention here.
Add a generic property test

This test is inspired by its namesake in GTK+. We instantiate
all types, and check the default values of their properties,
with some exceptions for types that are known not to work.
Fix a typo in a comment

The D-Bus interface is org.freedesktop.DBus.Peer, not
org.freedesktop.DBus.
GIOModule: Add some more details to the docs
Correct a doc comment
Improve GIOModule test coverage

Add a dedicated test for GIOModule.
Add tests for remote actions

This improves the GApplication test coverage.
Improve converter test coverage
Improve gdbus test coverage
Improve gsettings test coverage
Improve GResourceFile test coverage
Unify error reporting in gfileutils.c

Use the set_file_error helper function in more places,
saving some 50 lines.