Before this commit, the Alt+- shortcut reduced the heights of group
comboboxes in the main window and scan popup, but not their widths.
These comboboxes did not become narrower than at the time of their
construction.
Fixing the height of a group combobox to that of a line edit in
MainWindow's and ScanPopup's eventFilter() is still useful after this
commit - to avoid the ugliness of a few pixels-higher combobox.