Switch to UTF-8 strings internally

[twanvl]

Currently MSE uses wxString everywhere. This makes it hard to interact with any other library.

The proposal is to switch to use utf8 encoded std:string for all strings internally, and only convert to wxString for the UI functions.

Pros:

• Less dependence on wxWidgets
• We can use a move constructor

Cons:

• Lots of work for little gain

Issues:

• wxString has a stupid implicit conversion constructor that uses the current locale instead of always utf8. Solution: make a wrapper class.
• We have to be careful about broken strings (ending in the middle of a utf8 codepoint). Solution: validate input upon reading a file.
• We need an iterator that walks over codepoints.
• boost::regex doesn't actually work with utf8 strings, so "." would match a byte and not a code point, which is very bad. Solution: Use boost::basic_regex<UniChar>, and codepoint iterators.
• Should we be using C++20 std::u8string?