[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:
{F8276897}

{F8276896}

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

src/declarativeimports/plasmaextracomponents/qml/PlasmoidHeading.qml

@@ -65,6 +65,7 @@ import QtQuick.Templates 2.12 as T
 
     background: PlasmaCore.FrameSvgItem {
         id: headingSvg
+        visible: fromCurrentTheme
         imagePath: "widgets/plasmoidheading"
         prefix: location == PlasmoidHeading.Location.Header? 'header' : 'footer'
     }