Build plumbing for allowing applet config pages to omit internal margins
Right now all applet config pages automatically get margins and this can't be disabled. Most of the time this is fine. But sometimes, you do want to disable them because the page itself wants to have a frameless style, with an inner list view that touches the edges of the window and sidebar. Currently this can't be done.
With this commit, the infrastructure now supports pages registering
that they want to opt out of automatic margins by setting
includeMargins: false in their config.qml files where they define
their config pages. They will then be responsible for adding any needed
margins themselves.