UI|Client: Animate widget opacity when enabling/disabling them

It is good to avoid abrupt visual changes.
UI|ToggleWidget: Ensure a disabled toggle’s state can be seen

The “flipper” of the toggle is now drawn with a larger-than-one
alpha so that it will be opaque even if the widget as a whole has
transparency.
UI|Client: Keep task bar hidden until app startup is complete

Looks cleaner this way.
Cleanup
Performance|Renderer|Refactor: Retain constness in APIs; avoid redundant memory allocs

It is important not to accidentally lost constness of object
references, particularly when dealing with Qt containers. Non-const
access to containers may force Qt to detach the data into a separate
private instance.

However, the most impactful change is the removal of the QBitArray
in Rend_RadioBspLeafEdges(). Allocating and freeing the array in
complex maps is much slower than simply calculating dot products
for the planes as we go.
Merge branch 'master' into resourcesystem

Conflicts:
doomsday/client/src/render/rend_fakeradio.cpp