main/postmarketos-base-ui-gnome: new aport and cleanups

In the process of reviewing the postmarketos-ui-phosh package, I found that many cleanups can also be done.

Marked as Draft because I am not done and some things might deserve greater discussion. Mainly:

• Looking at the git blame, maybe ambient-enabled=false should go into the pinephone specific package? By discussion, don't consider it pinephone specific.
• dconf profile should be removed and configs installed as overrides. Maybe in 00_postmarketos-ui-phosh.gschema.override or maybe in their own files

Also, I would really recommend reviewing this commit by commit, detailed explanations are included in each of them.

Closes #1362 (closed)

Relates #1347 (closed)