[Feature Request]: Press enter when adding fields

[poseidon-rises]

Checklist

• I am using an up-to-date version.
• I have read the documentation.
• I have searched existing issues.

Description

When adding a new field to a file, add the ability to press enter and add the field without pressing the Add button.

Solution

Allow the user to press enter when the Add Field window is open, which does the same as pressing the Add button.

Alternatives

The user could use the cursor to press the button, but this is undesirable for 2 reasons:

1. The cursor is slow, and most users prefer to use their keyboard
2. Many other programs allow the user to press enter in similar menus, so users assume it's a feature

[CyanVoxel]

Added in #941