Rework font sources UI

[ollimeier]

Fixes #1997

• I use "add-remove-buttons" (similar to other areas of Fontra) to add or remove a font source, this makes the repeating trash-icon redundant
• When adding a new font source, this will be 'selected'.
• With arrow up and down you can go to previous or next source (this will be ignored if the focus is for example on an input)

[ollimeier]

Short demo:

Screen.Recording.2025-02-04.at.15.46.28.mp4

[justvanrossum]

It mostly works well. If I use the arrow keys, and the list of sources does not fit, the selection can get out of view. You should use scrollIntoView.

I have not done a thorough code review yet, but left one comment.

[justvanrossum]

Visual feedback: I would like the elements of the source panel to be more left aligned, instead of floating in the middle.