D22886: [kcmkwin/kwindecoration] Slightly improve tab bar's integration with frame

ngraham created this revision.
ngraham added reviewers: KWin, VDG.
Herald added a project: KWin.
Herald added a subscriber: kwin.
ngraham requested review of this revision.

REVISION SUMMARY
  The QML Window Decorations KCM uses a fake tab bar with code copied from Plasma-pa, due
  to the lack of a real QML tab widget (see https://bugs.kde.org/show_bug.cgi?id=394296).
  As a result, we have to manufacture our own by combining a TabBar with a frame in which
  the content sits, and any change made to one needs to be applied to the other as well.
  
  The Plasma-pa version just received a minor change in D22533 <https://phabricator.kde.org/D22533>. This patch implements the
  same change for the Window Decorations KCM so they don't look inconsistent with one
  another.

TEST PLAN
  Before-and-afters are identical to the images in D22533 <https://phabricator.kde.org/D22533>

REPOSITORY
  R108 KWin

BRANCH
  slightly-improve-fake-tabbar (branched from master)

REVISION DETAIL
  https://phabricator.kde.org/D22886

AFFECTED FILES
  kcmkwin/kwindecoration/package/contents/ui/main.qml

To: ngraham, #kwin, #vdg
Cc: kwin, LeGast00n, fmonteiro, sbergeron, jraleigh, fbampaloukas, GB_2, mkulinski, ragreen, jackyalcine, Pitel, iodelay, crozbo, bwowk, ZrenBot, ngraham, alexeymin, himcesjf, lesliezhai, ali-mohamed, hardening, romangg, jensreuterberg, abetts, sebas, apol, mart