Fix and optimize StatusbarProgressWidget

See commit messages for details.

I prepared a patch to test the optimization in the second commit: analyze-connectSingleItem.patch.

When I opened KDevelop with several small projects open (fully parsed on the previous run), its output ended with:

singleItem == mCurrentItem 25
!(singleItem == mCurrentItem) 4

When I run a Find/Replace in Files search, the output ended with:

singleItem == mCurrentItem 489
!(singleItem == mCurrentItem) 8

After a while of working on KDevelop inside KDevelop built with these commits, the output ended with:

singleItem == mCurrentItem 14609
!(singleItem == mCurrentItem) 80