AbstractKCM: make footer top padding optional
Right now, AbstractKCM has explicit logic to add extra top padding to an empty footer. This is done so that unframed QML KCMs without footers look correct when opened in System Settings, where the System Settings shell app itself adds buttons on the bottom of the window that require the extra padding to be added in KCMs to look good.
However ths extra padding is not required when a QML KCM using this component that has an empty footer doesn't have any bottom buttons added by System Settings itself. An exmple is the Bluetooth KCM, and the GHNS dialog also uses it.
It's not possible to automatically detect when this automatically-added padding won't be needed so we can omit it. As a result, the only way to let implementations opt out of it is by making it configurable using a new public property.
We should get rid of this in KF6+Plasma 6 and do it properly by having System Settings add the padding to its own footer, but this isn't possible now because it would break too many other things due to differences in the Plasma and Frameworks release schedules.