Straight forward port. Note: this is currently crashing deep down in
QtQuick. To circumvent the crashes it helps to disable the property
highlightFollowsCurrentItem in the listviews. This solves the problem
that QtQuick crashes on first loading. Unfortunately it still crashes
if one tries to invoke TabBox for the second time.
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
due to plasma change, shadow is not included by default, hence kwin
tabbox need to handle shadow by itself, and due to kwin is compositor
and have shadow as effects, it's not a good idea to use X property to
pass shadow from kwin to X and to kwin. This change use the old way to
draw shadow side the tabbox, and partly move the blur region setting to
qml.
REVIEW: 108243
