Move packaging to containers and add windows-msi and linux-appimage

[provokateurin]

Moves all packaging to Docker containers and adds full support for Windows MSI and Linux AppImage installers.

[pchampio]

Please separate modifications into separate PRs:

• Docker needed for containers
• The move of functions into internal packages
• The addition of packaging Windows MSI

I'm myself not the best at choosing what's must be put into PRs.
A good rule of thumb is on PR -> one modification idea (either it is a feature a fix...).

[provokateurin]

I understand what you mean. But would only remove the MSI packaging part and leave the others in this PR

[pchampio]

@jld3103 please wait form #33, I don't want to rebase twice.

hover started as a small project, more and more feature got added in top of the others, and I may confess, the code quality doesn't match the one on go-flutter.

I like the fact that you started moving function into separate files, but there is a lot of things that needs to be discussed first, I don't necessary like the overuse of internal package.
And after a quick look, I spotted some regression.

I've done some light clean-up in #22.
It adds a convenient fileutils.CopyTemplate that replaces the for loop used to write in files.

regarding moving packaging to containers, same as above, it needs discussion.

What doesn't needs discussion is windows-msi and linux-appimage.
It will be much easier if you opened a PR containing:

1. package.go split into multiples files.
2. linux-appimage
3. windows-msi

[provokateurin]

Yeah agree. Just have to look how I can separate the code changes. And yes, I think the code quality is not that great.