GBM remote access support for KWin

Summary:
Implements a KWayland protocol to pass GBM fd from KWin to KRfb and
addictions to relevant projects from both sides.

Note that this patch does not affect default behaviour of mentioned projects. It can be used
only with KWIN_REMOTE=1 in env from KWin side and with preferredFrameBufferPlugin=gbm in krfbrc from
KRfb side. In all other aspects app behaviour remains unchanged.

Test Plan: Launched KWin in Wayland mode, launched KRfb in it, launched KRDC on a laptop, connected in read-only mode, observed a correctly retrieved desktop with Krfb window

Reviewers: graesslin, davidedmundson, romangg, #kwin

Reviewed By: davidedmundson, romangg, #kwin

Subscribers: kossebau, jgrulich, romangg, ngraham, alexeymin, aacid, kwin, #kwin, davidedmundson, plasma-devel

Tags: #plasma_on_wayland, #kwin

Maniphest Tasks: T5653, T7785

Differential Revision: https://phabricator.kde.org/D1230