Use translucent/dialogs/background elements where possible

In effects it's obvious that compositing is enabled, so specifying the
translucent element is no problem.

In tabbox a context property "compositing" is injected which decides
whether "translucent" or "opaque" elements should be used. Here the
translucent elements are only used if the Blur effect is available - for
this a new Effect::Feature Blur is introduced and in addition it is
tested whether the theme provides the translucent element.

Also the masking is adjusted to ensure that only the shadow is not
blurred.

Reason for this change is that Plasma theme seems not always to pick up
whether compositing is used when used from inside KWin. It does not cover
the Desktop Change OSD which uses PlasmaCore.Dialog and there we cannot
(yet) inject that we use compositing.

Overall I'm quite unhappy with this patch and I do hope we can fix it in
the proper place in the lifetime of 4.10 and revert this patch.

CCBUG: 311995
REVIEW: 108438