Implement GtkApplication based activation and uniqueness

This replaces the existing dbus-glib activation and uniqueness code.
The changes are kept to the minimum necessary to make all the features
work, but there are still some optimizations possible (like doing most
of the initialization in ephy_application_startup() when we know we
are not remoting). These changes are left for later to avoid making
this patch huge.

Command-line parameter parsing is done in the main method and
parameters are passed to the application through a
EphyApplicationStartupContext structure, which is later passed as a
GVariant to the primare instance. This way we avoid moving the
GOption code out of the place where it's intended to run: in the
main() method.

Based in work by Alexandre Mazari.

https://bugzilla.gnome.org/show_bug.cgi?id=637334