The current layout of the qrc editor GUI is confusing. The add button is overly complicated. The property panel always has items disabled. Also the tree view with the file names takes most of the horizontal space which is unnecessary.
Suggestion:

• Tree view with the prefix + file names on the left, spanning most of the vertical space, with "Add Prefix", "Add Files" and "Remove" buttons below it
• To the right of it show a properties panel for the current selection that contains only the applying properties, e.g. "Alias" for files, "Name" for prefixes, Language for both
• The properties panel could contain an image preview for image files
• Maybe drag and drop to rearrange files