preliminary Diff implementation for #230 (draft)

[Fat-Zer]

TODO:

• [ ] wxStaticText resizing (and text rewrapping) should be reimplemented.
• [ ] Add settings for colour of diff text background in UI (people with dark UI theme will likely want to change those).
• [ ] Add context menu option to copy old source text.

Note: It assumes dtl is installed system-wide during build. Feel free to change it if you are going to package it with tarballs.

[vslavik]

OK, I think I give up on trying to make static text work; as referred to in the other issue, the only path forward seems to be to use read-only wxTextCtrl as you suggested initially.

[cooljeanius]

does this PR support msgmerge's --previous flag?

[cooljeanius]

My previous comment was not off-topic; msgmerge's --previous flag is an existing way for marking differences between translation strings that should be considered when implementing a new diff implementation, such as this PR attempts to do.