[MacPorts] #55092: meld: bump to 3.18.0 (python3)

[MacPorts]

#55092: meld: bump to 3.18.0 (python3)
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 Reporter:  elelay    |      Owner:
     Type:  update    |     Status:  new
 Priority:  Normal    |  Milestone:
Component:  ports     |    Version:
 Keywords:  haspatch  |       Port:  meld
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 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.

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Ticket URL: <https://trac.macports.org/ticket/55092>
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