NaCl and SDL2 are now hotswappable again without any unimplemented or
superfluous function parameters.
Also added missing EnumSet operators for Modifiers.
In SDL they are only for keyboard events and there is no support
for mouse modifiers like in X. Also reordered the enum in
AbstractXApplication to make it consistent.
Because with NaCl WindowContext doesn't make sense anymore, the classes
are now renamed:
* Contexts namespace -> Platform
* *WindowContext -> Application
Hopefully this is (one of) last crazy renaming runs.