[aurorae] Improve reading the border elements from C++ side

The KWin::Borders element is provided by an extension plugin. The reason
for that is to be able to use it from e.g. the kcm or Plasmate without
needing to compile the code in.

But this results in Aurorae itself not being able to access the element.
The solution is to first load our decoration plugin and afterwards
register the borders element again with the version compiled in from
Aurorae.

With that we can now read all borders and paddings without using
properties. Also we could connect to change signals and have the borders
and padding handling completely stateful. Might be an idea for extending
the decoration library...