Translatable strings for UIs and developer tools are mixed together

Since libadwaita is going to be used all over GNOME now, we’ll need to make it a bigger priority for translators. Before we do that though, I believe the strings should be split into two sets, exactly as GTK already does: one for strings that will show up in the UI, and one for “developer” strings. This way smaller teams can complete interface translations and not be forced to work with almost 200 highly technical strings that almost no one will see.