Introduce a dedicated borders element in Aurorae

The Borders element provides the four properties:
* left
* right
* top
* bottom

And is used directly in Decoration for all the different kind of settings
following this pattern:
* normal borders
* maximized borders
* padding
* extended borders

These properties replace the existing used borderLeft & co. This makes
the code in the C++ side easier as the various border elements can now be
read with a shared implementation.

The Borders provide some convenient methods to set the sizes of the
borders. E.g. it's possible to just set the side borders to a specific
value. This should simplify the implementation of the no-side-borders
feature in new decoration.

The aurorae qml and plastik are adjusted to use the new way. Existing
3rd party decorations would break, but there's a good reason why there's
no documentation for QML based decorations ;-)

REVIEW: 108436