Fixed|Documentation|Doxygen: Incorrect source file names
UI|Client: Removed old Control Panel, settings now in the Appearance editor

Added a new panel-derived widget called FoldPanelWidget that supports
embedded, folding groups. This is used for settings groups in the
Renderer Appearance editor.
Fixed|libdeng2: IndirectRule's value becomes zero when unset

When the source rule of the indirect rule was removed, the indirect
rule did not invalidate itself. Instead, the last value before the
unsetting remained in effect.
UI|Client: Improved shared interface for console variable widgets
UI|Client|GridLayout: Allow defining a fixed width for a column

The fixed width defined for a column is used in the layout calculations
without affecting the size of the widgets in the column.
UI|Client: Widget improvements

- Panel eats mouse clicks not handled by children.
- FoldPanel closes immediately.
- Slider dots made slightly dimmer.
UI|Task Bar: Removed "Control Panel" from the config menu
UI|Renderer Appearance: Reseting individual setting groups to defaults

Each foldable group of setting can be reseted separately. The default
values of the cvars were added to ClientApp's
rendererAppearanceSettings.

Fine-tuned the alignment of groups so that the first column in
each uses the same width.