Like how we're handling activation, use GVariant for timestamps. To
avoid polluting the GtkApplication API with GVariants, we rename the
GApplication signals to "quit-with-data" and "action-with-data".
GtkApplication will then wrap those as just "quit" and "action".
https://bugzilla.gnome.org/show_bug.cgi?id=621002
This adds a GApplication object to GIO, which is the core of
an application support class, supporting
- uniqueness
- exporting actions (simple scripting)
- standard actions (quit, activate)
The implementation for Linux uses D-Bus, takes a name on the
session bus, and exports a org.gtk.Application interface.
Implementations for Win32 and OS X are still missing.
