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.

[Fat-Zer]

rebase upon current master and bundle dtl into tarball (there is little reason in keeping source-only library separated)