Move app delegate's source icon into the action button

Right now ApplicationDelegate shows an icon for the source for apps from a non-default source. However the icon appears floating in the corner of the delegate rather than inside the button to install or remove the app. This was done as a workaround for https://bugs.kde.org/show_bug.cgi?id=433433 but feels kind of awkward.

That issue is not yet resolved, but there is an alternative: write a custom content item for the button to add the missing feature. This lets us put the icon in the button as originally intended without beeding to wait for a fix in the framework.

This commit implements that change.

simplescreenrecorder-2021-07-17_12.31.29

cc @apol @teams/vdg