Added support for multiple mouse button events

[ElliotGarbus]

This PR adds support for multiple mouse button events, fixing issue #3597
The PR also adds some documentation for how the mouse multi-touch simulation works.

Implementation note: The original implementation used a variable, self.current_drag to hold the current touch. This limited the behavior to only work correctly when one button was pressed at a time. In multi-touch simulation is this not an issue as the buttons have predefined roles. When multi-touch simulation is disabled this did not allow multiple buttons to be used simultaneously. This fix changes self.curent_drag to a dictionary, and holds a touch per button. In the case that multi-touch simulation is enabled, a single key is used to access the self.current_drag dict, retaining the original behavior.

When multi-touch simulation is disabled. Each mouse button will properly report their button up/down state changes. If the mouse is dragged with multiple buttons pressed there will be an on_touch_move message for each button.

Maintainer merge checklist

• Title is descriptive/clear for inclusion in release notes.
• Applied a Component: xxx label.
• Applied the api-deprecation or api-break label.
• Applied the release-highlight label to be highlighted in release notes.
• Added to the milestone version it was merged into.
• Unittests are included in PR.
• Properly documented, including versionadded, versionchanged as needed.