[MacOS] Add ci mode to build and create dmg file

Goal

Add a new "CI mode" to build and create .dmg file using system side dependencies.

It's the only way we found to make a "cleaner" .dmg build and add a minimum of changes in the gajim.spec file.

Thanks a lot to @niyabits for her tests, debugs and solutions!

What this PR do

• In gajim-macos-helper.sh script:
  • Use the master branch of gajim and nbxmpp by default
  • Add CI mode to build and create .dmg file
  • Add the CI_BUILD variable
  • Move python dependencies in a variable at the top of the file
• In gajim.spec file:
  • Add hidden_libs variable to add libsoup and libgtksourceview
  • Define the good Homebrew library path according to system arch
  • Use a glob to select the longest path to find hidden libraries and add them in our dmg bundle (without specifying the version number, so it will work on all MacOS and even after upgrade of this libs we hope)
  • Use a glob to collect GI repository typelib files
  • Ignore PIL._imagingft to avoid adding wrong non-system versions of dependencies libs of PIL._imagingft
• In README.md:
  • Explain the new CI mode

Information

• Fix #9713 (closed)