Fixes GTK packaging on Linux

[crsib]

• I signed CLA
• The title of the pull request describes an issue it addresses
• If changes are extensive, then there is a sequence of easily reviewable commits
• Each commit's message describes its purpose and effects
• There are no behavior changes unnecessary for the stated purpose of the PR

Recommended:

• Each commit compiles and runs on my machine without known undesirable changes of behavior

[LWinterberg]

While this fix seems to not apply the GTK theme properly (maybe it's some sort of fallback/default? Certainly isn't adwita-dark), it's way more usable now. Testing across the board of linux UI bugs,

• Icons missing from dialog windows #2279 is fixed
  
• No Visible Scrollbar in Plugin Manager #3543 is fixed
  
• Linux: No scrollbars #2222 and
• Linux: Missing horizontal scrollbar #2937 aren't really fixed: The scrollbar functionality appears invisibly as soon as its needed, but the visuals only update when the window is resized. This also applies the other way round, if you have scrollbars and then delete your tracks, white bars will stay around the edge where the scrollbars used to be.
• Linux: No file or folder icons in file browser #2270 is fixed. While there is a single icon shown as an x, I believe this to be a case where my icon pack doesn't specify that icon, rather than something Audacity related.
  
• Text on Input and output drop-down lists go out of frames #2624 is mostly fixed
  
• Linux: Spectral Selection Toolbar malformed #1800 and
• Linux: Text too large in Spectral Selection Toolbar #1456 aren't really fixed - even at 100%, text gets cut off ever so slightly, compared to other dropdowns. At 200% it completely runs off (see below)
  
• Linux: Clicking on track slider fails to display value #1709 is fixed for me.
• HiDPI issues #503 does not change (not that I expected that it would). Here's an example of Audacity inside a properly behaved app.
  
• Linux: Interface issues with GTK3 #1536 appears fixed at 100%, at least I don't get the bugs listed inside it as examples ("some elements are malformed, notably the Device Toolbar appears to be empty, and text in the track's control panel is tiny.")
• Linux: Toolbars malformed when not docked #1609 is fixed (probably: never was broken) on 100%
  

I think that's all of them. I don't know how many of them should be marked as resolved by this PR :)

[crsib]

Whoa, that is quite a lot of feedback, thank you!

[crsib]

I think #1536 is about GTK3 build, we are still building GTK2. So not fixed, I guess