Add comment to core

[Arthur-Milchior]

The main goal of this PR is to ensure that all strings get a comment. While I used to naively think that most string were clear, and that translators would be able to figure out what they were translating, I realized, looking at the French translation, that it's not actually the case.

Instead of just fixing the French translation, I prefer to improve the process so that AnkiDroid can be correctly translated in all languages.

In particular, some strings are used in multiple context. E.g. as a button and the title of the dialog/view opened by this button. In English, it's certainly correct. In French it usually is too. I have no idea whether it's correct in all other languages. By letting the translators know that the strings is used in both context, they can provide feedback if ever they need us to split the string in two.

As an example, I recently split "Search" in two strings, one for the verb and one for the noun "search", because, in French, we need to use distinct words to consider the search action, and the content of the search request.

This problem would not even be solved by looking at the screenshot. Indeed, while it's easy to see which strings has no screenshot, it's not possible to state which usage is not screenshotted. So the translator may not have seen that "search" was used in two distinct contexts.

I should note that this task requires to look at all the strings, and look for all its usage in the codebase. Which means this task can only be done by a developer that knows the codebase and the app enough to understand all usages of each string. Worse, either reviewing such changes will be very painful, or the reviewer will have to have a high trust in the person doing this work. Which means, and I'm quite sad about it, that it can't really be delegated.

As a secondary goal, the commit reorders the string, so that they strings appearing in the same view get shown together. This is not probably not important for translators, but I find it cleaner.

I don't exactly expect this PR to be merged as-is. I mostly open it to gather feedback. If you are against this change (e.g. because it'll destroy the history of all our string files), I can stop and not spend more time on it.

Or, if you have any idea of a better way to write comment, I can use your idea in the future comments.

Once we agree this work is worthwhile and can eventually be merged, if you find typos, gramattical errors, or general improvement, I'd appreciate if you could directly push the correction as an additional commit, or provide the correction as a suggestion in github review system. Because, discussing every comment individually would be a nightmare of bikeshedding