Support "Save As" for custom editors

[msujew]

What it does

Closes #14966

Allows Saveable instances to implement their own saveAs routine. This is required to fix saving for custom editors. All other editors can continue to just serialize their contents to use the generic "Save As" function.

How to test

Follow the reproduction steps of #14966 and ensure that the "Save As" functionality works for both the .pawDraw and .cscratch files.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[VadimGroGit]

I'm sorry, I'm an absolute noob and I don't know the procedure for including patches in the Theia build.

I was hoping to see

msujew/fix-custom-editor-save-as

in the February build, but as far as I understand, this is not the case.

I did a branch checkout and checked - SaveAs works for both CustomEditors, text and binary.

When can I expect to see this branch included in the main release?

Or are there some problems with fix?

[msujew]

@VadimGroGit Thanks for looking into it. This is a less impactful bug/fix, so it's likely nobody had the opportunity to review it in the meantime. @JonasHelming care to assign someone?

[rschnekenbu]

@msujew, reading the code and testing, I did not find any issue. I could also reproduce the steps given in references issue. Thanks!