Select design or text view mode for new items

Summary:
This is mainly for scripts: they do not support design view and, when
trying to create a new script it failed, with “!window, switching to
view mode failed, "Design View"” because KexiMainWindow always opened
new items with that mode.

I tried to create new items of all types and they seem to work, as the
only that did not like design mode was scripts.

However, i wonder if KexiParts should not have a “new item mode”
attribute or something similar to avoid hard coding it in
KexiMainWindow.

FIXED-IN:3.2.1

Reviewers: staniek, piggz

Reviewed By: staniek

Subscribers: Kexi-Devel-list

Tags: KEXI

Differential Revision: https://phabricator.kde.org/D29243