strings: Rename "application" to "app"

This change is part of an initiative to use the word "app" instead of "application" in the UI:

Teams/Design/initiatives#123 (closed)

Also, this MR drops the period from the appchooser description text, because a period isn't necessary in phrases with only one sentence: https://developer.gnome.org/hig/guidelines/writing-style.html#periods

GTK appchoser MR: gtk!5323 (merged)