Part of the refactor into the config system.

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.

GraphicsSlider is used by the panes in the Graphics config window to
create sliders 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 ConfigSlider better reflects its
purpose. This should also make it less confusing when ConfigSliders are
added to other config panes.