cursor: Avoid seat round trip for handle_pointer_*

Another split out from the gesture work:

We connect the signals in the seat while disconnecting them in the seat for no good reason. So move those over to the cursor avoiding a set of public funtions.

We want to do the same for touch and tablet later on to fullyfix #231 .

We can also have the cursor emmit an "activity" signal later on so the wlr_idle_notify_activity() can move to the desktop but I've postponed that until handle_touch_* and handle_table_* moved over too. (I've skipped touch since that would cause additional friction with gestures and skipped tablet since I remember @dos talking about some pending focus/grab rework there).