Use the same code for editable and non-editable buttons

Use the same code for editable and non-editable buttons

This removes the use of rectText in RenderButton. The fact that this
gave different spacing than buttonText was a problem.

Now buttonText requires to specify
* the offset, so that INSET_TO_TEXT_OFFSET is not used anymore in
  src/frontends/, which will be useful later.
* the background and frame color, in replacement for the hover state.

Remove the methods button() and buttonFrame() from GuiPainter.

Remove some unused header files.

Fixes bug #10704.