Before this change syncthingwidgets unconditionally included the header
from libsyncthing so it couldn't be used as stand-alone library if
libsyncthing was disabled.
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.
Martchus