Support buildaction for C++ projects

[TurkeyMan]

Had to remove 'allowed', since the list of possible build actions is extensible and kinda arbitrary.

[samsinsane]

LGTM, should this still validate the C# input? Over in the csproj code, rather than _premake_init.lua?

[TurkeyMan]

Yeah, it probably needs to result in None if a bad action name name was given.

I think C# also has an arbitrary/extensible set of buildactions just the same as C++ no?
One lame thing about the C# build actions is that the existing list of actions are MAPPED to the real action names. I think C# should accept the real action names, and not re-name them as it has.

Also, I think the C# code should really be refactored into rules, just like the C++ project code.

[TurkeyMan]

It's already handled: https://github.com/premake/premake-core/blob/master/src/tools/dotnet.lua#L63
If buildaction is given some value that isn't supported, it may end out with None, which is what the new C++ implementation does.
That said, there's also a bunch of other logic and matching going on in that code for other conditions, but it wouldn't be right to change any of that.

[TurkeyMan]

I'll update the wiki to document the removed 'allowed' fields when this merges.

[TurkeyMan]

Amended with a unit-test, cus I'm a stylish guy like that ;)