Various picking bugfixes

[NthTensor]

Objective

• Intended to resolve Only single Pointer<Click> when multiple hits against Pickable::should_block_lower:false #15290.
• Fix four duplicate DragEnd firing when drag finished.
• Fix redundant DragStart firing when dragging across pick-able entities.
• Fix Click coming after Drop and obliterating finished drag interactions.

Big thanks to B. Reinhart for testing picking in their codebase and identifying these issues early.

Solution

• Fix press & drag state being cleared after the first entity is read from the hover map on pointer release, rather than after all entities are read. This caused only the first hovered entity to receive Up and Click events.
• Fixes Down being determined using the previous_hover_map rather than hover_map, a regression compared to bevy_mod_picking. I think this is what was messing up drag events.
• Fixes and issue where PointerEnd would fire multiple times and PointerStart would fire when dragging onto a new entity.
• Re-orders events to make them easier to handle. Out now fired before DragLeave and Click/Up now fire before DragDrop.
• Generally refactors the picking event code to be more clean and sane.

Testing

These changes are currently sporadically tested.

[alice-i-cecile]

This has been tested by @brandon-reinhart, and is at least an improvement on the current behavior. See these messages.

[NthTensor]

I'll be using this PR to just collect up a big batch of fixes so we can merge them all at once.

[alice-i-cecile]

@NthTensor say the word and I'll merge this in.

[NthTensor]

LGMT