Fix compilation with KIO < 5.33

KURLRequester's acceptMode property has been introduced in KIO 5.33 (but
Kexi's minimum supported KF5 version is 5.16.0).
With older versions, the KUrlRequester's fileDialog() has to be modified
directly.

To be able to do this depending on the KIO version, set the property in
the class constructor instead of the .ui file.

Differential Revision: https://phabricator.kde.org/D19616