Let us use consistent horizontal size policies in scan popup's and main
window's GroupComboBox and TranslateBox. MinimumExpanding in scan
popup's TranslateBox allows it to stretch horizontally along with
horizontalSpacer when the width of popup window increases.
Before this commit, when group combobox was visible, the Alt+- shortcut
reduced the heights of group combobox and translate line in the main
window, but not in scan popup. These UI elements in scan popup did not
become lower than at the time scan popup was constructed.
Applying the solution from the main window fixes this bug (#1108).
There is no point in having huge margins (9px) at every side of the
scan popup window. Especially since the popup window is supposed
to be small and compact.