[kwin] Introduce a X-KWin-Internal in kwineffect services

X-KWin-Internal should be used for effects which are either internal
to KWin and are configured at a different location in KWin. Examples
are:
* CoverSwitch - configured in WindowSwitcher KCM, but not FlipSwitch
as that one has a non WindowSwitcher mode
* window geometry - configured in moving

And it should also be used for helper effects to the overall
kde-workspace module. Examples are:
* Dashboard - required by Plasma
* KScreen - required for KScreen
* Screenshot - required by KSnapshot

Why a new category and not the already existing NoShow property?
The idea is to just filter on the effects in the list. Thus it should
be possible to show them. But NoShow is clearly intended to not be
shown at all.

REVIEW: 116754