[Keba] Implement thing action to set display text

[maihacke]

This PR is related to the suggestion in #18025.
It implements a new thing action for the keba binding which allows to set the display text:

rule "Set Display Text"
when
  System reached start level 100
then
   val keContactActions = getActions("keba", "keba:kecontact:1")
   // Default duration
   keContactActions.setDisplay("TEXT$1")
   // Explicit duration set
   keContactActions.setDisplay("TEXT$2", 5, 10)
end

Regarding QS I created a version for 4.x and tested it. Works quite well. There are now code changes in 5.x, and in the end it uses the existing infrastructure of the binding to set the display text. So assume it will work in 5.x also.