Feature to import/export .ent files

[Outerbeast]

Will obviate the need for using ripent alongside bspguy
Options to import/export only brush entities or point entities

[wootguy]

UnrealKaraulov added this a while ago but it's not in the latest Windows build.

[UnrealKaraulov]

UnrealKaraulov added this a while ago but it's not in the latest Windows build.

Sorry but I no have time to recreate my pr, I still add some more features and fixed in my fork, like support "angles" keyword, etc.

[UnrealKaraulov]

@wootguy also, https://github.com/UnrealKaraulov/bspguy/blob/86f22f20c587698ac17b745507139f59540064b9/src/editor/Gui.cpp#L257 , now all exported BSP models has working collision, even client support this models without any problems.

[UnrealKaraulov]

But need skip optimize "clipnodes", because collision is broken after optimize clipnodes.

And I added FileManager in my fork, but this feature required c++17. (Filesystem)