Implement tooltips that show the type of an output port on hover.

[MichaelMauderer]

Pull Request Description

Implements #960.

Important Notes

• There is an issue with text occlusion that needs to be addressed in the future. Right now there seems to be no good way to allow us to specify relative occlusion of text in the same way as shapes.
• The tooltip cannot be part of the cursor because it would create cyclic dependencies: the cursor lives in enso-core, but the tooltip requires enso-text, which in turn depends on enso-core. Thus, it is implemented as a standalone entity in the graph editor. A more appropriate place might actually be next to the cursor in the Application, but the same issue is present there.

Checklist

Please include the following checklist in your PR:

• The CHANGELOG.md was updated with the changes introduced in this PR.
• The documentation has been updated if necessary.
• All code conforms to the Rust style guide.
• All code has automatic tests where possible.
• All code has been profiled where possible.
• All code has been manually tested in the IDE.
• All code has been manually tested in the "debug/interface" scene.
• All code has been manually tested by the PR owner against our test scenarios.
• All code has been manually tested by at least one reviewer against our test scenarios.