Prevent keyboard modifiers from leaking

• Instead of carelessly allocating Modifier objects on the heap and forgetting about them, turn the list of all modifiers into their owner.
• Replace all static maps with static local variables of helper functions to partially guard against the static initialization fiasco.
• Improve const correctness of the Modifier class
• Small improvements in the code.

With this fix, 2 more unit tests now pass on a sanitized build.

Not addressed: the ugly design with a global state tweaked by the Shortcuts system.