Refactor|Client: Added SettingsRegister, renderer & input settings

SettingsRegister provides several important functions:

- Stores/provides access to the default values of settings.
- Hides differences between old-style cvars and new script Variables
  when it comes to persistency.
- Manages a set of alternative, user-customizable profiles that
  each have an independent set of values for the settings.

There will be one SettingsRegister per domain.

Note that SettingsRegister augments and builds on the normal
persistent Config (and con_config).
Refactor|Client: Applied SettingsRegister for audio and video settings
UI|Client: Video Settings fullscreen mode, Close button

The "Mode" setting was replaced with a "Fullscreen Mode", which
only affects the fullscreen mode.

Adding a Close button makes Video Settings more consistent with the
other dialogs. The "Color Adjustments" button was moved into the
dialog to make room on the dialog button row.