[Plasmoid Heading] Draw the heading only when there is an SVG in the theme

Summary:
We're the only theme with the appropriate heading svgs that make the headers and footers look as they should.

All the other themes fall back to Breeze, which is not a good look for some of them:

• the heading is too jarring compared to applet background
• the heading does not extend all the way vertically and horizontally

To solve this, to avoid degrading unmaintained themes and to give themes a chance to adjust on their own terms, this patch makes the heading visible only when the needed SVG exists in the theme.

Test Plan:
Avoids this:

Breeze still works as it should.

Reviewers: VDG, Plasma, niccolove, ngraham

Reviewed By: VDG, niccolove, ngraham

Subscribers: ngraham, niccolove, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D29352