Add missing translation keys

[ollimeier]

Fixes #1763 and fixes #1752

This is the first part of the rework for missing translations.

I split it up, because it may get messy otherwise. This PR includes commits based on each file (Related Glyphs, Glyph Note, Shortcuts panel, Sources Panel and Status Definitions panel). But the next work might not be structured this nicely anymore, because the strings are all over the place.
So, I created another branch where I first marked the missing translations:
https://github.com/googlefonts/fontra/tree/issue-1752-missing-translation-keys-2

[ollimeier]

@justvanrossum I am wondering if we really want to have a unique translation for undo (always)?

Here an example:
We have undoLabel: "align left", this would require a new translation key, but we have already a translation for 'Align left' via key: sidebar.selection-transformation.align.left.
This is same for many other undo strings. It's basically just set in 'lowercase'.
Do you think it would make sense to reuse some translation keys and accept it's not lowercase? We could also make it always convert to lowercase, but I am not sure if this would work (always) for other languages except English?
But doubling the keys also does not feel correct.

Would be great to get you feedback and advice. Thanks a lot in advance.

[ollimeier]

As discussed yesterday, this translation key update is almost complete, but we decided to leave it as it is, so that we do not spend too much time on translations and can focus on other features.