Don't resize window when switching virtual desktops in OpenBox

It looks like switching virtual desktops in OpenBox invokes a show event
(this doesn't happen while running Plasma).

The code in MainWindow::showEvent() should only be run once on first show,
so guard it with a bool member.

BUG: 441610
FIXED-IN: 21.08.1
(cherry picked from commit 68f1505d)