Call updateScripts in KReportDesigner::setScriptSource

Summary:
KReportDesigner already has a slot that calls this function when the page
button is clicked. However, both in the example application as well as in Kexi,
the page is already selected when showing the designer for the first time and,
unless the user presses the page button again, trying to select a script from
the combo box does not display any choice, not even “none”/empty, and it prints
the following warning on the console:

org.kde.kproperty: NO SUCH KEY: "" property= "script"

Reviewers: staniek

Reviewed By: staniek

Subscribers: staniek

Tags: KReport

Differential Revision: https://phabricator.kde.org/D28762

FIXED-IN:3.2.1