Self-argument arrows: New edge rendering mode

[kazcw]

Pull Request Description

Implement the new edge-rendering mode for #9064. The mode is not used yet, but can be tested by hard-coding targetIsSelfArgument to true.

^{Demo of the new edge mode. Note that when this mode is put into actual use, the arrows will point to the icons.}

Important Notes

• For ease of review, changes have been divided into a preparatory refactor commit and the new functionality.

Checklist

Please ensure that the following checklist has been satisfied before submitting the PR:

• The documentation has been updated, if necessary.
• Screenshots/screencasts have been attached, if there are any visual changes. For interactive or animated visual changes, a screencast is preferred.
• All code follows the
  Scala,
  Java,
  and
  Rust
  style guides. In case you are using a language not listed above, follow the Rust style guide.
• All code has been tested:
  • Unit tests have been written where possible.
  • If GUI codebase was changed, the GUI was tested when built using ./run ide build.