Add command line switches to disable pointer and touch

I am using a wireless keyboard with an integrated touchpad.
I do neither need nor want the touchpad, but I am unable to
attach the keyboard only (without the pointer device) to my
computer. I therefore find it convenient to simply disable
that device when starting `cage`. The switch for disabling
touch input is there for consistency. Maybe it comes in handy
to someone else at some point.

I don't have any previous experience with wayland, so there is
a chance that I simply missed some environment variable that
accomplishes the same. In this case, this commit would be
unnecessary.

The command line switches `P` and `T` are quite ad-hoc. I probably
would have gone for `--disable-pointer` and `--disable-touch` if
long command line switches had already been in use. If someone has
a better idea than `P` and `T`, please suggest it.

server.h

@@ -47,6 +47,8 @@ struct cg_server {
 
 	bool xdg_decoration;
 	bool allow_vt_switch;
+	bool disable_pointer;
+	bool disable_touch;
 	enum wl_output_transform output_transform;
 #ifdef DEBUG
 	bool debug_damage_tracking;