Pointer events support

[toasted-nutbread]

This change adds support for pointer events, which enables custom scanning modifiers for pen devices.

This change works well on Chrome, but has some issues on Firefox, which are presumably browser bugs:

• Pen pointer up/down events register as touch events instead of pen events. over/out events register correctly as pen.
• The above bug also causes the keyboard-mouse input fields on the settings page to not detect pen buttons correctly.
• Pen buttons also are not even detected correctly when buttons are held. On Chrome, they do work as expected.

The events incorrectly registering as touch instead of pen can probably be worked around, since the pointerId is the same for both. This will be done as a separate change later.

Originally mentioned in #267. This feature is disabled by default.