Add support for primary, secondary and middle mouse buttons. Also improve ability to click things in low FPS situations.
This introduces a lot of breaking changes:
Backends/integrations now pass mouse events via the even stream.
Response has an interface of mostly methods instead of public members.
input.mouse is now input.pointer and has new interface.
* Rename 'mouse' to 'pointer' everywhere (pointer = mouse or touch)
* Make Response::clicked and Response::double_clicked into methods
* Remove Response::active and add dragged() and interact_pointer_pos()
* Support multiple mouse buttons
* Make PointerState interface all methods
* Make most members of Response private
* Wrap Layout and Region into a new struct Placer
* [egui] Add a simple grid layout
* Refactor CollapsingHeader code (simplify header painting)
* Fix: allow putting a CollapsingHeader inside of a grid layout
* [demo] Add a widget gallery
Closes https://github.com/emilk/egui/issues/88
* Add optional striped grid background
