Get rid of stored GdkEvents in Canvas

This is another low-risk part of !4959 (closed), which involves simplifying the canvas code responsible for keeping track of the current item.

• The main goal is to eliminate the use of GdkEvent variables inside the canvas to store information (_pick_event). In this case the information had become redundant with another variable anyway (last_mouse).

• The _in_repick variable is removed. It guarded against recursive picking, which is not something that should be happening anymore.

• A refactoring improvement is pulled over from !4959 (closed): split pick_current_item() into find_item_at() and repick().

• Finally, various dead code from some other event-related files is deleted.