Creates a layer outside the game config layer system and passes it to the created gfx widows, so as to not interfere with the global config system.

Supports multiple game properties being open at once.
Supports editing while a game is playing, but the options only save and update the active game when the window is closed.
Right-clicking will remove a property from the game ini.

This reduces code duplication in the different ConfigControls. This is
helpful for the next commit, which will modify the now deduplicated
code.

GraphicsRadioInt is used by the panes in the Graphics config window to
create radio buttons that change their associated config setting, and
update their own state when something else changes the config setting.

Despite its current name nothing about this class is particular to the
Graphics window, so renaming it to ConfigRadioInt better reflects its
purpose. This should also make it less confusing when ConfigRadioInts
are added to other config panes.