fix LuaConsole completion list reset to happen when text is changed
internal functions should be static
remove Gui::TextEntry::onFilterKeys, which was only used by the console

With the transition to SDL2, text input for the console is now filtered
with a hack in the main event loop. In the future, it should be filtered
in the UI event dispatcher based on whether there a text-accepting widget
has the focus, and it should possibly use SDL_StartTextInput and
SDL_StopTextInput.
slightly simplify the console-switch input filtering hack
flush TEXTINPUT events whenever a Gui::TextEntry widget grabs focus
SDL_Renderer doesn't need to be foward declared as it isn't used

(and SDL_Window doesn't need to be forward declared because we make the SDL
API available everywhere anyway, which is perhaps not ideal but nevermind)
Merge remote-tracking branch 'johnbartholomew/sdl2-fixes'
make WHEELUP/WHEELDOWN constants into enums for older compilers