Desktop: Add File > Save As…

[timon-schelling]

Closes #2973.

• Improve save document semantics
  • Removes name from values saved in document (No migration should be needed)
  • Instead uses file name (without .graphite) as the document name
  • Document name is updated when the document path is updated
  • Document path is reset if the name is change by the user
• Add Save as action
  • Adds Save As Menu entry and shortcut
  • Issue: Save as shourtcut Ctrl + Shift + S not working (help wanted)

[TrueDoctor]

@Keavon We fixed the open issue this is now waiting on your thoughts regarding how the save process flow should work and if the implemented behaviour is desired

[Keavon]

Let's discuss in a call so I know more about what's to be considered.

Also, this PR has a conflict.

[TrueDoctor]

@Keavon what is the status on this? I can review the code you just have to say if you are happy with the behaviour