gdbus-codegen: Fix leak in property setter

Comparing the code generated for the setter and other methods without
(real) return value, I noticed that the setter does not unref the
gvariant it gets.

https://bugzilla.gnome.org/show_bug.cgi?id=719472
gio: Replace g_clear_object() with if()…g_object_unref()

As per https://bugzilla.gnome.org/show_bug.cgi?id=113075#c39.
Test new gsettings api

g_settings_get_user_value and g_settings_get_default_value
are now covered.
Improve test coverage for gsettings
Improve test coverage for GSubprocess
Improve test coverage for GHashTable
Improve GKeyFile test coverage
Improve GNotification test coverage
Improve search utils test coverage

This adds test cases for the Turkish i.
Improve GSettings test coverage

This covers some more GSettingsSchemaKey api.
Generate overview images for coverage

lcov has the options to generate overview images for each source
file with the --frames option. Lets try this.
Improve GSimpleActionGroup test coverage

Add a case that overwrites an existing action.
Improve GNotification test coverage
Improve GApplication test coverage

Exercise more of the GActionMap implementation.
Add a test for g_atexit
Improve GType test coverage

Add a test for g_type_class_get_instance_private_offset
tests/gsubprocess: Only compare basename of /tmp

Otherwise, we break in Continuous where /tmp == /sysroot/tmp,
and something in the execve() or the kernel will do realpath()
for us.
Add a test for g_file_measure_disk_usage
gtestdbus: Fix variable shadowing

Shut up, GCC.

https://bugzilla.gnome.org/show_bug.cgi?id=712148
gtestdbus: Fix non-const use of const variables

The argv array should be declared as const.

https://bugzilla.gnome.org/show_bug.cgi?id=712148