Chore: Create poptracker.dmg

[Virunas]

• Created a simple disk image file to move the poptracker app to applications since it was on the TODO list.

[black-sliver]

What's meant by the item on the TODO list, is adding a script to build the DMG for each release

• in the Makefile
• and/or as a script in macosx/
• and/or as part of the build automation in .github/workflows/

or maybe just documenting how to build one yourself as part of BUILD.md.

If you could transfer the steps you did to build the DMG to one of those things, that'd be appreciated.