applets/systemtray: don't continuously reparent popups between panel and popup
The initial parent of every systray applet fullrepresentation is the preloadStorage Item.
Applets fullrepresentations are pushed in a StackView, which on pop restores the old parent they had before pushing, which is preloadStorage. this meant the applet fullrepresentation kept changing scene every time the popup was opened and closed, which is note great for performance.
As a side effect, if a ToolTip was open, due to the reparent, the tooltip found itself in the new window and shown inside the panel.
Move preloadStorage in the popup, so that full representations are never moved between windows