Create upgrade path to transition from 3rd party apps

[TraceyC77]

This is necessary for #72

Create an upgrade path for people using 3rd party apps that have a flatpak available.

Also account for whatever we do with mscorefonts getsolus/packages/issues/6161

[Justinzobel]

A small discussion just happened in Matrix regarding this, transitioning notes to here.

1. We create/repurpose the MATE transition tool for this project (more on this in step 3)
2. We post in the forums advising that 3rd party is shutting down in x time (a month for example). This post contains information on each app, and its Flatpak link (for those that have it as well as a small guide on how to move the config/data to Flatpak directory). Those that don't have a Flatpak, we create a package.yml for and indicate that these are provided as is and will not be updated. The community can potentially rally around supporting these on non-Solus infra, e.g. like we had a git repo where people submit updates.
3. A month after the forum post, we replace each third party packages with the 3rd Party Transition Tool (from step 1) that takes the place of that app on their system. This app would show a prompt to the user to explain that app x is no longer available and that 3rd party repository is no longer maintained. It has options to uninstall the program or install the flatpak (where applicable).

[joebonrichie]

Related: getsolus/solus-sc#315

[TraceyC77]

Since the team decided to forgo transitioning apps without a flatpak alternative, we only need to consider the ones with a flatpak available.