EK bar opacity

[knyipab]

Implement EK bar opacity, for #605 .

[RalfWerner]

@knyipab, can I get an artifact from you, if you've success?
With your version from "actions" the text "under" the EK (50%) is visible, but the fingers, stylus, mouse control are not possible. Only touchMode Trackpad allows both. Finger Control applies than to the EK.

various checks with extra keys

[knyipab]

@RalfWerner true that (even with EK bar opacity), non-trackpad still have to deactivate EK bar to access the bottom area. Still, it provides visibility.
@twaik I don't hold strong view between this PR #627 opacity and PR #618 adjust height. It's good to at least have a choice for user in the preferences page.