shell/scripting: add imageSource to containment
This allows to get the screenshot of a containment from DBus scripts.
Test: run /usr/lib64/qt5/bin/qdbus org.kde.plasmashell /PlasmaShell org.kde.PlasmaShell.evaluateScript 'print(desktopsForActivity(currentActivity())[0].imageSource)'
or /usr/lib64/qt5/bin/qdbus org.kde.plasmashell /PlasmaShell org.kde.PlasmaShell.evaluateScript 'print(desktopsForActivity(currentActivity())[1].imageSource)'
Edited by Fushan Wen