Fix ABI issues

bump .so number of kdecorations2private

A new virtual was added, so we need to bump the version number. This
means rebuilding kwin, but not any 3rd party decorations

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Restore ABI of libkdecorations2.so

requestShowWindow() was removed. Restore with an implementation that
sends an invalid QRect so that we don't need two versions throughout the
private API as well. A little bit hacky, but it should suffice
well-enough for deprecated code.

This should be ABI compatiable with the existing released
libkdecorations2.so.5