Prevent the "Nothing in register x" error linger.
This can be illustrated by trying to paste from an empty buffer in vimode (e.g. "zp)
or by setting 2 marks fast enough (e.g. in vimode, qqmambq@q@q)
For now, this can only be practically illustrated using the 1st method as 2 KTextEditor::Messages are always posted when pasting from an empty buffer (one from modebase.cpp and one from insertvimode.cpp)
Before
After
The other minor change is that a-z buffers are now always shown in lowercase