Summary:
this avoids some unwanted scene changes and speeds up the
first open a bit, also initialize the size of the dialog main item
so there is a resize less when the Dialog is first shown

Test Plan:
difference in show time is already visible, tough still not perfect,
something else needs to improve as well

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: apol, broulik, hein, davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10692