ZynAddSubFX/FLTK: fixed detection of freetype2 include path once more

Commit 26665dde55cfa589f166998ea587db70cd516b68 was not complete as we have
to find the Freetype package and also look for freetype2 include path.
AutomatableModelView/ContextMenu: option for unlinking all controls

Integrated patch by Rodrigo Rodrigues da Silva which adds an option
to all AutomatableModelView's context menu allowing to unlink all
linked controls.
AutomatableModelView: adopted new coding style