On-screen keyboard should support setting/clearing of modifiers, not just toggle

[mike.jumper]

Currently, the OSK always toggles modifiers if a key is defined as a modifier. This makes implementing modifier keys whose behavior changes based on other modifiers awkward.

Each cap should be able to explicitly set an arbitrary set of modifiers, clear another set of modifiers, and toggle yet another set of modifiers.

Until this is implemented, it is only practical to define modifiers which are always available and completely independent.