System tray popups are currently resizeable through means of an obscure method (ALT + right click + drag) and the resized values are not remembered on plasmashell restart.
This patch attempts to implement a sidebar mode, which is just the popup expanded to maximum available height.
WIP because it doesn't work well for when the panel is on the bottom or right (see vid) and because the mode is not remembered after plasmashell restart.