filter plugins and do not modify doc if plugin was removed

also introduce BINDINGS, to be used shortly
introduce BINDINGS

- remove duplicates also for bindings
- added cmake macros/functions:
 - remove_binding
 - list_filter
 - list_find
- fix configure-debian to use BINDINGS=ALL
- handle non-existance of interpreters in gi_ (do not simply fail)
- allow to add individual gi_
- remove deps if glib is not available
- fix PYTHON2_LIBS (2 was missing)
remove options obsolete because of BINDINGS

- also fix constants to output correct list of BINDINGS
- keep PATH and CPATH for LUA
remove tools if deps not present

- use PYTHON2INTERP_FOUND correctly
- do not make qt-gui a subproject