meld: bump to 3.18.0 (python3)

[elelay (Eric Le Lay)]

Following #47560 and #51512, here is a new attempt at upgrading meld.

Attached is the updated Portfile.

Installation of data files (esp. glib2.0/schemas) is necessary directly in prefix, not in Frameworks/Python, otherwise there is a crash at startup and some icons are not found. This could be worked around, but since a single version of meld will be active at one time (no pyxx-meld subports), it's better to keep it simple.

I've added the application construction. It works nicely with gtk quartz but is untested on X11.

• Application and command-line work,
• UI localization and icons work,
• only tested Python 3.4, not 3.5 3.6, though it should be compatible.

Debugging tip: modify /opt/local/bin/meld:264 to log.setLevel(logging.DEBUG) instead of CRITICAL.

@mf2k, I'm willing to help maintain meld, since I use it often, but won't be able to test it on High Sierra.

[elelay (Eric Le Lay)]

diff against 2a94add

[mf2k (Frank Schima)]

Please just support the latest python 3.6 and remove the older versions. I can test it on High Sierra.

[elelay (Eric Le Lay)]

In e80bec0273eea0f31a6416c2a3b0d3353b6a1c4d/macports-ports:

meld: update to 3.18.0 (python3)

• adopt it
• move yelp dependency to variant
• make an application

closes: #55092

[elelay (Eric Le Lay)]

Done, thanks. Please let me know if it breaks on High Sierra.