Odd seat <-> cursor split

The seat has a cursor however the cursor's listeners aren't connected in it's module but rather in the seat module but some of them are then proxied over to the cursor while others (e.g. handle_tablet_tool_position) are handled in seat.c directly.

Looks as if the whole handle_cursor_ handling for wlr_cursor events could be moved over into cursor.c and then just have the cursor emit a "activity" signal so the seat can emit wlr_idle_notify_activity().

This would also get PhocCursor way closer to not expose all of it's members in the header.