OSX - 3.19.2 (r1) - Mojave

This release adds tons of features. It is thus a bit "beta"

1. Upstream update to meld master (3.19.2)
2. Now uses built-in python3
3. Core build dependencies updated to latest that jhbuild offers. (newer GTK)
4. Adds support for Mojave Dark theme
5. Much smaller in size
6. Seems to solve the saved-state issues that faced people using previous version of Meld for OSX made for High Sierra