Have recipes, branches, and major versions depending on the major version of elementary OS

[ryonakano]

Problem

I think we currently focusing on supporting the latest version of Pantheon Desktop because:

• we have only one recipe on Launchpad for multiple major release of elementary OS (Focal is for 6.x and Jammy for 7.x)
• and we have only one branch main for release

However, if we update the code to follow the latest major version of elementary OS, sometimes it may causes a breaking change or removes functions that still available in the previous versions, e.g:

• FilesPane: Remove single-click option #66
• Remove VideosPane #120
• FontsPane: Fix icon missing with the latest icons update #129
• Port to GTK 4 #163 (this is not merged yet, but System Settings would migrate to GTK 4 in some major release of elementary OS, which is a completely breaking change for older versions)

For now we're only supporting the one version of elementary OS―6.x―after renaming to Pantheon Tweaks, but 7.x would be out at some point and then maybe it would be better to support both of them because not all the elementary users prefer "the latest is the greatest", right?

Idea

So, I think we should prepare to support multiple versions of elementary OS. For example,

• Create a branch per major version of elementary OS.
  • e.g. 1.x for elementary OS 6.x and 2.x for 7.x, …
  • Pantheon Tweaks is also ported to another distros, so maybe we should not use versions or code names of elementary OS here.
• Create a recipe on Launchpad per branch on GitHub
  • This is what elementary does; for Files, they have files-juno, files-focal, etc.
  • So we create pantheon-tweaks-jonior for elementary OS 6.1 and pantheon-tweaks-horus for 7, for example.
  • Maybe we can use versions or code names of elementary OS here since PPAs are designated for Ubuntu-based distros
• Define that bumping major version of Pantheon Tweaks means is a big thing
  • Let users consider that bumping major versions of Pantheon Tweaks may change the supported version of elementary OS; e.g. Pantheon Tweaks 1.x supports elementary OS 6.x, 2.x supports 7.x, etc.

[ryonakano]

Closing in favor of #187