Fix async rendering of map item
Summary:
Retry the render while items are downloading
Notify via a signal when async rendering has finished
Test Plan: Using POI.kexi and a clean cache of marble, open the report
Reviewers: staniek
Reviewed By: staniek
Subscribers: Kexi-Devel-list
Tags: KReport
Differential Revision: https://phabricator.kde.org/D3028