UI|Client: Added MessageDialog; relocated rest of updater sources
UI|Client|SequentialLayout: Conveniently adding spaces
UI|Client|ProgressWidget: Added a "mini" style of progress indicator

This is intended for notification widgets.
Default Style: Define rules based on units

Makes it easier to scale the entire UI, e.g., for high-DPI displays.
UI|Client: Minor widget tweaks and improvements
Refactor|Updater|UI|Client: Updater moved to ClientApp; revising dialogs

The Updater is now a proper C++ class and part of ClientApp. The
old C APIs were dropped.

The update notification dialog was reimplemented with new widgets.

Todo: Download dialog.