[ShellGlobal] add SHELL_STAGE_INPUT_MODE_FOCUSED

Add SHELL_STAGE_INPUT_MODE_FOCUSED, to move the keyboard focus to the
shell without grabbing the keyboard or mouse, and make
stage_input_mode into a GObject property so that (among other things),
callers can tell when MODE_FOCUSED reverts back to MODE_NORMAL.

https://bugzilla.gnome.org/show_bug.cgi?id=623429

src/shell-global.h

@@ -51,6 +51,7 @@ void shell_global_grab_dbus_service (ShellGlobal *global);
 typedef enum {
   SHELL_STAGE_INPUT_MODE_NONREACTIVE,
   SHELL_STAGE_INPUT_MODE_NORMAL,
+  SHELL_STAGE_INPUT_MODE_FOCUSED,
   SHELL_STAGE_INPUT_MODE_FULLSCREEN
 } ShellStageInputMode;