Cleanup
Client|Updater: Relocated some autoupdater header files
libdeng2|IndirectRule: Allow evaluating an undefined indirect rule

The value of an undefined indirection is zero.
Refactor|libdeng2|Widgets: Notifications about child addition/removal

Using the observer mechanism for notifications about child addition
and removal is more flexible and results in fewer virtual methods
in all widgets.
Refactor|UI|Client: Data item semantics, variant data, custom sorting methods

The semantics of UI data items are now expressed as a set of flags
that can be combined as necessary.

All UI data items can contain a QVariant as user data.

The data context can be sorted with a user-provided comparison
function.
UI|Client: Separate margins, opacity for disabled widgets

GuiWidget now supports a separate margin rule for each edge of the
rectangle. Presently LabelWidget applies all of them correctly.

The opacity of disabled widgets is now automatically reduced.
Fixed|UI|Client|ChoiceWidget: Several improvements to ChoiceWidget

- The font of the selected item defined in the Style.
- Fixed addition of items into the widget.
- Popup can open to the right.
UI|Client|GridLayout: Cell alignment can be set for each column

Columns can be either left-aligned (the default) or right-aligned.
Only affects variable-width columns.
UI|Client|DialogWidget: Apply common settings for contained widgets

Widgets added to a dialog's content area get automatically styled
with appropriate settings.
UI|Updater|Client: Reimplemented the updater settings dialog

Now using Doomsday's own UI framework for the settings dialog.