AbstractOutput is not so Abstract and it's common to avoid the word
"Abstract" in class names as it doesn't contribute any new information.
It also significantly reduces the line width in some places.
The .clang-format file is based on the one in ECM except the following
style options:
- AlwaysBreakBeforeMultilineStrings
- BinPackArguments
- BinPackParameters
- ColumnLimit
- BreakBeforeBraces
- KeepEmptyLinesAtTheStartOfBlocks
The output management test checks the implementation of output
management capabilities in the virtual backend, which is not helpful.
This change replaces it with a more useful test that verifies how
windows are placed after an output change.
