Refactor|UI|Client: Dialog action button placement to opposite edge

Dialog action buttons are now placed in a separate menu on the
opposite side of the dialog. However, there is only a single buttons
data model for a dialog, shared by both widgets.

The Gear button in UpdateAvailableDialog is now displayed using this
mechanism.
Default Style: Added a 'gauge' icon

This is useful for developer instruments, data inspection, and other
tools.
Default Style|Task Bar: Added a set of basic settings icons

Some of these are a little draft quality...