Add glib, pango, and pixbuf tags. Drop JESSIE. Default BUSTER.

[thebeline]

This upgrades the Golang version for STRETCH to the latest supported, downgrades the BUSTER tag for gtk to avoid a weird linker issue I discovered in golang1.16 that may not be necessary, but also doesn't break anything, and ADDS various supported tags for glib, pango an pixbuf.

Now, you may be wondering why these changes are needed. Well, they don't break anything, and they WILL be needed if and when we upgrade to the latest version of gotk3. I already have a working branch with gotk3 in it, I am just not pulling ALL of those changes in one go (for sanities sake), and instead I'll give you some foundational things first.

Mazel

Resolves: #270

[thebeline]

Also includes security and release updates:

• golang1.15 for STRETCH
• Drop JESSIE support
• Default to BUSTER (latest Raspbian base since 2019-06-24)

[thebeline]

Resolves: #277

[thebeline]

I should note that lowering the build tags for gtk from gtk_3_24 to gtk_3_22 is, again, to avoid some issues with the latest gtk and the latest gotk3 when combined with golang1.16. We don't USE golang1.16 at this time, but in case we do in the nearer future, this prevents that issue. It is something we can re-visit raising again later, when we do migrate to golang1.16, but at this time it is a safety measure, and I can assure you that it does not effect anything we use (note how the flag was set as low as gtk_2_14 for JESSIE, and that built just fine.

That particular change was just to err on the side of caution, and triggers no changes in the functionality of the app.

[thebeline]

@JeffB42 - How should I proceed with this? Does this still seem relevant to you?