This reverts commit eb0e82a7.
It's causing different deadlocks, notably when a thread tries to
remove a handler inside a handler.

This should bring things to the mostly-regression-free-but-easier-to-debug
state I was expecting when I removed DIBs from many bitmaps.

comctl32/tests: Change expect_band_content test to ensure line numbers are reported correctly on test failure.

In particular, if we'd unregister a window when the application replaced our
window proc, but still forwards to us, we'd create a loop when we register
the same window again later.

Multi-character character constants aren't portable as their behaviour
is compiler dependent according to the C standard (sparse).