Make resizing of piano keyboard possible (without mouse wheel)

So everything is said in the title of the issue. There is currently no way to resize piano control without using mouse wheel (+ctrl key).

Often wheels on mouses don't work too good and sometimes they doesn't work at all:)

My suggestion is to add some keyboard shortcut for that (like ctr + Plus which works for zooming scores - it looks consistent for me). Also to make the borders of the "piano control" window resizable - that's a natural way to change the size.