UI|Default Style|Client: Color adjustment dialog; cvar slider; style tweaks

The color adjustment dialog is for the monitor color adjustments.

- CVarSliderWidget presently works only the float cvars.
- Added an option to signal on Enter key press in LineEditWidget.
- Fixed an issue with PopupWidget reparenting (popup should always
  become the last child of the root to be on top).
- Right-click on SliderWidget allows editing the value as text.
UI|Client: Added the color adjustment dialog in ClientWindow

ClientWindow owns a color adjustment dialog, and it can be shown at
any time.

The ranges of vid-gamma, vid-contrast and vid-bright were adjusted
to be within more reasonable limits. The CVarSliderWidget uses these
ranges directly.