Extended mouse button support in canvas actions

Revamp of this old task.

First patch fixes some grammar blemishes in libs/ui/input.

Second adds the ability to bind all 28 exposed mouse buttons in Qt to canvas input actions, up from the 5 currently supported.

I tracked down at least one upstream bug rebasing these, so there may be more hiccups around some devices. This time I at least have twice as many tablets to test them against and besides that Qt regression they work and that Qt bug seems to be the origin of a lot of the gripes in the original submission.