Make play button clickable and hide it when irrelevant

[Procrat]

Pull Request Description

Closes #6177: Clicking the play button should show a spinner icon. The play button should only be visible on execution environments that have one or more execution contexts disabled.

play-button.mp4

Important Notes

There's a bug in the existing code that's exacerbated by this PR: the clickable area for the drop down menu is not directly placed on top of the text. Michael is fixing that in a separate branch (39ce6a2).

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.

[vitvakatu]

QA: passed