Switch to go mod and latest go, improve build

[the-ress]

I plan to contribute some fixes, but first I wanted to make the building and development experience a bit more pleasant.

Main changes:

• Switch to go modules
• Update to latest go - 1.19
• Add cross compilation support (so I can build arm packages on my laptop)
• Add automated github builds

Necessary changes for the above:

• Update gotk3 to master due to [BUG] Fails to build under Go 1.16 gotk3/gotk3#726 and then [BUG] glib support does not build on older versions anymore, because of G_FORMAT_SIZE_BITS gotk3/gotk3#811
• Some minor source changes to get it to compile after the gotk3 update
  • For example, here's one of the changes in current gotk3: Refactor internal closure handling and several API changes gotk3/gotk3#706

    Old Func types for callback setters no longer force a variadic list of arguments in their parameters, as Go's closure abilities allow referencing values from outside.

• Drop jessie (deprecated, no go 1.19 support)

Aside from the gotk3-related changes, I didn't touch any .go files so I hope it won't collide with any other work that's going on. (Bulk of the changed files is the deleted vendors folder.)

I also tried merging these changes with 2.8 - there's only 1 very trivial conflict and it compiles fine.

[thebeline]

Hmmm, interested to see the differences between this and #270

[the-ress]

@thebeline
I looked at them and #346 does everything #270 does (and much more - switching to go modules, cross compilation, automated github builds).

There are some minor differences in tags - I didn't need to add the gdk_pixbuf_ tags and I needed glib_deprecated and cairo_1_14 that aren't included in #270. I also added BULLSEYE target (because why not).

Here's the Makefile diff between #270 and #346: https://github.com/Z-Bolt/OctoScreen/compare/cddd776..c165467#diff-76ed074a9305c04054cdebb9e9aad2d818052b07091de1f20cad0bbac34ffb52

[the-ress]

Changed target do 2.8.0-dev

Update: Resolved conflicts and rebased to current 2.8.0-dev

[JeffB42]

@the-ress Thanks for the contribution. Just a heads up here... this PR probably won't be going into 2.8 (there are over 1,200 changes w/this PR, and I'm close to releasing).

Also, for releases after 2.8.x, I'm considering dropping GO and building OctoScreen as an Electron application using Typescript. I'll need to spike on this first, but the more I use GO and GTK, the less I like it.

[the-ress]

Also, for releases after 2.8.x, I'm considering dropping GO and building OctoScreen as an Electron application using Typescript.

That's... unexpected. I'm worried it'll tank the performance, especially on boards like RPi Zero 2. Go is actually why I chose OctoScreen over OctoDash (OctoDash is using JS+HTML and was way too slow when I tried it.)

Edit: It's even noted in README:

[TouchUI] requires a browser to run, which consumes a lot of resources. This is the main reason OctoScreen was developed as an X application.

[the-ress]

there are over 1,200 changes w/this PR

Most of that comes from the deleted vendors directory. The actual changes are like 36 files and most of that are one-liners to make the code compile with newer gotk3.

[JeffB42]

That's... unexpected. I'm worried it'll tank the performance,

I will be keeping that in mind. It's not a sure thing, and I will be spiking on it later before making a decision.