libdeng2|Widget: Renamed potentially dangerous method

Widget::clear() is now clearTree(), as clear() might be confused with
clearing the widget's own contents rather than the entire tree
under it.

Also, clear() might be confused with clearing a UI data context --
the end result is visibly similar (child widgets gone, however
the corresponding data items still remain).
Refactor|UI|Context|Client: Modified removal audience, API fluency
UI|Client|DialogWidget: Non-modal dialogs

DialogWidget can now also be opened in a non-modal manner.
UI|Client: Various widget improvements; cleanup

ProgressWidget's default mode is to align content normally rather
than by image only, as the latter is only really needed in
BusyWidget.

LabelWidget determines the maximum available text width better:
the constraints set on the label image must be taken into account.

PopupWidget constrains the placement of the popup so it doesn't
go outside the view with the Down opening direction.
Refactor|UI|Updater|Client: New Download dialog for the Updater

The download dialog is attached to the updater's notification widget,
and can be dismissed to continue downloading in the background.
Updater|UI|Client: Usability improvements; cleanup

The game is automatically paused when a background download completes
and the dialog is shown.

Added the option to abort the installation of an update even after
the download has finished.