Always adjust IntMapComboBox to contents

Summary:
By default combobox adjusts to its contents the first time it is shown.
IntMapComboBox is used, for example, to show available speeds to burn,
which are added to the combobox after it is shown, and thus can be
displayed cropped.

Reviewers: aacid

Reviewed By: aacid

Differential Revision: https://phabricator.kde.org/D26209