Rebrand as Selfie

This MR replaces all (I think) user-facing instances of the word “Cheese” with “Selfie”. It does not rename any files, C/Vala namespaces, executable names, app IDs, etc. (This is consistent with, for example, Epiphany being branded as Web.) Rough consensus on #13 seems to be that Selfie is a reasonable choice. In Endless, we changed the .desktop file to use Selfie several years ago, following user testing where users were asked to take a photo of themselves, and several of them did not find Cheese.

The first patch in this series adds "selfie" and "camera" to the .desktop file keywords – even if we do not change the app name, or change it to something other than selfie, I think both of those are reasonable keywords to add. Happy to split that to a separate MR if preferred. !26 (merged) was merged and released in 3.38.0.

How best to handle translations?

In Endless, we have a number of translations of the word “Selfie” in the desktop file:

Name=Selfie
Name[ar]=سيلفي
Name[de]=Selfie
Name[es]=Selfie
Name[fr]=Selfie
Name[hi]=सेल्फी
Name[id]=Swafoto
Name[pt_BR]=Selfie
Name[th]=เซลฟี่
Name[vi]=Ảnh tự sướng
Name[zh_CN]=自拍
Name[zh_TW]=自拍

Several of these are transliterations of the English word “selfie”. I checked all of these against the titles of the article corresponding to https://en.wikipedia.org/wiki/Selfie in each language's Wikipedia, and they are consistent, with the exception of some transliteration variation, and in the case of Spanish, the article being titled “Autofoto” (though it lists “Selfi”, “Selfy” and “Selfie” as alternatives).

However I think this is best left to the discretion of each GNOME language team – at the end of the day, it is their decision. (It looks like a majority of translations keep the English term Cheese, though a few translate to something analogous.)