fs::path <> string interoperability

[dimitre]

this PR is to experiment interoperability between fs::path and string.
I think this was not happening before because of some systems were using
std::experimental::filesystem::v1::__cxx11::path which caused errors when converting fs::path to string.
I've updated ofConstants.h in this PR so __cxx11 is not being used anymore

[dimitre]

it will cause some test errors with stuff like
ofToDataPath(path).back() to get the latest character of a string, and it doesnt' work with fs::path
I personally think the gains outweight the potential differences.
the idea here is change return type to fs::path and remove the FS() versions
thoughts? @artificiel @ofTheo

[artificiel]

+1 for path return type everywhere.

for the failing tests they should be updated to explicitly perform the string conversion:

ofxTestEq(ofPathToString(ofToDataPath("movies\\",true)).back(), '\\', "absolute ofToDataPath with \\ should end in \\");

(incidentally it's more or less what someone holding to some legacy code using string methods directly on the return of ofDataPath might need to do, but method chaining is not prevalent in user code)

[dimitre]

Great. if we decide to proceed with this idea I'll complete the work in this same PR.

[dimitre]

I've finally finished this one. I've updated the tests accordingly.
This PR changes return type from a number of core functions from string to fs::path
but brings end to end fs::path to the core.

opinions? @ofTheo @NickHardeman @artificiel @danoli3

[ofTheo]

Wow. Awesome! Did you run into anything with the tests that might indicate breaking changes for old projects?

[dimitre]

In the tests the things I had to change is something like ofToDataPath("").back()
and my own usage the only thing I had to change is one addon using this line

	string resultado = ofSystem("open " + ofToDataPath(fullFileName));

I had to switch to something like:

	string resultado = ofSystem("open " + ofPathToString(ofToDataPath(fullFileName)));
or 
	string resultado = ofSystem("open " + ofToDataPath(fullFileName).string());

[ofTheo]

ahh so does that mean any code that has string + fs::path ( which used to be ) string + string, now needs something like: string + fs::path.string().

I guess that is the downside - but the upside is support for non English filesystems right?

[dimitre]

Yes exactly. support for different encodings end to end inside core.
I think two things are important for using OF for teaching:
being able to parse non english characters on path, and being able to work with spaces in paths.