radiant: update FGD parser (improvements from NetRadiant-custom)

radiant: update FGD parser (improvements from NetRadiant-custom)

Partially imported patches from:

- f0ce315ed3f8e826dad2a40e89bb7689c555f8bb
  * handle fgd spawnflags #115
- 62f06a9af34b70f1c3bfbdfb75b6405cdac8d4c5
  * appease incompatible fgds loading: only show message window once
- commit 8d19be1c0e506c6e3f9e5e2e7154ca81af077097
  * fgd: support studio("display/model/path") and flags(Angles)
- c6c978a38faebf7ad5cc1d240fe9c5e15491fc0a
  * load all found *.fgd, not just halflife.fgd

FGD spawnflags are not handled yet (only parsed and ignored).

Co-authored-by: Garux <garux@mail.ru>