Support for new decoration border sizes BorderBottom and BorderNone

BorderBottom is what is known from Oxygen as "No Side Borders". The name
should indicate that there is only a border at the bottom. BorderNone is
a mode with no borders at all.

The new enums are added to KDecoration and the KCM so that all decos can
make use of it.

Aurorae is adjusted to support the new sizes for QML themes (it breaks
the old svg based themes) and Plastik makes use of it, by rendering a
one-pixel border around the window, which illustrates that it's still up
to the decoration to decide how to make use of the setting.

@Hugo: in case you want to port Oxygen to the feature: due to ABI compatibility the BorderBottom and BorderNone are added to the end of the list, while in Oxygen they are listed as first elements (which make much more sense).

@All: a better name for BorderBottom would be nice :-)