Various issues with events

[GriffinRichards]

Some issues I noticed while working on the events API. Keeping a running list because I'll likely encounter more. All of these exist on the master branch and the most recent release (4.5.0).

• [ ] When the last event in an event group is deleted the tab for that group persists (unless it's the last event on the map). If the map is reloaded the tab properly disappears. If deleting the last event is undone with Undo, the event reappears but the tab does not.
• [ ] Numbers can be entered in an object event's "Sprite" field, but when redisplayed the number will disappear. Other text dropdown fields allow numbers to persist. Additionally, if the number is a valid graphics id the object's sprite is not displayed as such.
• [ ] Changing an event's z coordinate doesn't mark the map as dirty
• [ ] Saving after changing any of an event's properties (except x/y/z) doesn't remove the unsaved changes asterisk in the map title.
• [x] Pasting (but not duplicating) allows users to exceed the map limit on object events (Fixed, #442)
• [x] After pasting the first object event is selected instead of the newly-created event (Fixed, #442)
• [ ] Possibly a suggestion rather than an issue, but the selected event does not persist between tabs (e.g. select object event 5, switch to the Warps tab, then back to the Objects tab, and the selected object event has been reset to 1)
• [ ] Modifying an event's position by using the x/y spinner adds an entry to the event history, but can't be immediately undone. It seems to register after other entries are added in the edit history, or if the edit history is open and has focus.