So far the backend libraries' include paths were relative within this
repository. This means the header files could not be used at their
installed location.
This change replaces them with "<>" includes to fix that problem and adds
a new include directory so building everything at once still works.
With this change it should be easier to actually split some parts into
another repository if this one would become too big.
This is useful because sometimes the background color of the system
tray is dark but the overall system UI still uses a light theme.
For the Plasmoid this should never be the case, though. Hence the
extra settings are not available there.
Martchus