application: Only restore window size if setting says so

We'd fetch the to-restore window size from gsettings, which would be -1 x -1 if configured to not restore, but if there was already a window open, we'd ignore that value and replace it with the active window size. This somewhat defeats the purpose of the 'restore-window-size' setting. Fix this by only trying to restore a window size, no matter from where, if this setting is turned on (the default).