Instead of falling back to a set of default values or crashing the
window manager when an invalid mode is specified, check the value
of the ShellGlobal:session-mode property before taking over as WM
and make a clean exit if it cannot be resolved to an existent mode.
https://bugzilla.gnome.org/show_bug.cgi?id=676156
Rather than accessing global.session_type / global.session_mode
all over the place, delegate mode information to a dedicated
sessionMode object. While not very useful for now, we will replace
checks for a particular mode with checks for particular properties
that sessionMode defines based on global.session_mode.
https://bugzilla.gnome.org/show_bug.cgi?id=676156
