Client|UI|ProgressWidget: Defining a visual subrange for progress

At the moment, this is used in a sequence of busy tasks.
Client|Busy Mode: Replaced old busyvisual with a ProgressWidget

Much of the old busyvisual is now obsolete. The busy transition is
still drawn as before with the old code.

Todo: Needs cleanup and tweaking, restoration of the screenshot
background.
libgui|GLTexture: Taking over an existing OpenGL texture object
Client|UI|ProgressWidget: Adjusted shadow behind the wheel
Refactor|Busy Mode|BusyWidget: Transition screenshot moved to BusyWidget

It is now BusyWidget's responsibility to grab the screenshot taken
for the busy mode background and end transition.

BusyWidget draws the background while the UI is in busy mode.

Cleanup needed.
Client|Busy Mode: Cleanup

Removed a bunch of obsolete code.