Fix list placement of the character picker options for the characterpicker-plugin.

On screens with lower resolutions the list for picking special characters is cut off at the window borders.
This is because the cell size gets overwritten and the list is always placed at the characterpicker button position.
Readjust the character list placement and the cell size.
Merge pull request #1173 from rainerda/hotfix-0.24.x-rt56795-characterpicker-list-placement

Characterpicker list cut off at borders