FString string_view support, TextView member visibility

[russkel]

• add string_view constructor support to FString
• fix warning with unnecessary cast to size_t
• change some methods to protected so a widget that extends the TextView can reason about the scroll height/width. (in my case: in order to make the TextView always scroll down if it is already scrolled to the bottom extent upon append)

[gansm]

One of the patches uses the C++17 feature std::string_view. FINAL CUT was originally written in C++98 and ported from C++11 to C++14. So using std::string_view is not yet possible in the current codebase.

[russkel]

Would you find it acceptable to put kind of #ifdef check on the string_view features?

[gansm]

I have also added a constructor for std::wstring_view. Furthermore, std::string_view and std::wstring_view are now also tested in the FString unit test.

0d691f1