Redesign how keys are handled

Eekboard was made to resemble a physical keyboard, with modifiers and levels. Most of that doesn't map well to how on-screen keyboard operates as a text input method.

Keypresses and modifiers are mostly a legacy feature, limited to pressing enter, games and keyboard shortcuts. The main responsibility of a keyboard is to provide text. Different views replace levels, and buttons on different screens don't have to be married to each other like they used to with levels.

Current status: EekSymbol gutted. Much code spent on adhering to C calling convention :(

Remaining: Drop levels: merge EekKey with librs::Symbol, and split symbol levels (keys with many symbols) into views (many views with buttons with one symbol each) in the XML parser and renderer.