D5747: add pid to plasma window management protocol

[Eike Hein]

hein added a dependent revision: D5755: Expose PlasmaWindow::pid through 
WaylandTasksModel..

REPOSITORY
  R127 KWayland

REVISION DETAIL
  https://phabricator.kde.org/D5747

To: sebas, #plasma, hein, graesslin
Cc: davidedmundson, plasma-devel, #frameworks, ZrenBot, spstarr, progwolff, 
lesliezhai, ali-mohamed, jensreuterberg, abetts, eliasp, sebas, apol, lukas