Revisit mouse event handling

[hansihe]

Right now things are hacky and several things don't work properly, including:

• Passing correct mouse buttons to the surface
• Mouse handling when button is clicked while over a surface and dragged outside of the surface. Will most likely be done by adding gesture handlers to the PlatformView?
• Correct input frame handling. This will most likely require some state machine or something on the dart side (similar to how the Android platform views do it) in order to reconcile the impedance mismatch between how wayland vs flutter structure their events.