Eliminate background app and Deskbar app checks

This is a follow up on the fix for #9632.

Now that the group list in Deskbar never deals with background apps or the Deskbar app itself we can simplify the code by eliminating the checks, especially in Switcher.cpp (Twitcher).

Checking for background apps and Deskbar has also been eliminated from TExpandoMenuBar and TTeamMenu. The single point of entry for these checks is in TBarApp::AddTeam().

In Switcher.cpp remove OKToUse() since the list is assumed to contain only valid entries. TSwitchManager::CountVisibleGroups() also got removed because all groups are visible. TSwitchManager::_FindNextValidApp(), TSwitchManager::QuitApp(),  TIconView::ItemAtPoint(), TIconView::ScrollTo(), and TIconView::FrameOf() all got simplified significantly.