Move tag area to EtTagArea object
Move file area to EtFileArea object
Drop explicit call to gtk_main()
Remove unused code when closing preferences dialog
Remove the cancel button from the preferences

As settings changes are applied instantly with GSettings, a cancel
button is not useful.
Move dialog setting saving to application window
Implement hiding of file header widget

Show the file header information if the file-show-header GSetting is
true.
Avoid warnings when reading and writing ID3 tags

https://mail.gnome.org/archives/easytag-list/2014-July/msg00000.html
Remove use of deprecated GtkMisc xalign property

Use GtkWidget halign property instead.
Use GtkWidget popup-menu signal in EtLogArea
Use popup-menu signal in EtLoadFilesDialog
Use GtkWidget popup-menu signal in EtBrowser
Avoid warnings when updating the scan dialog
Use same strings in GSettings schema and UI

While not all strings are applicable to the UI, try to reduce translator
effort by reducing the delta.
Move some GApplication handlers to EtApplication
Refactor about dialog handling

Fetch the parent from EtApplication.
Split off main() to a separate source file
Remove last uses of deprecated GtkStock
Remove use of deprecated GtkImageMenuItem
Move tagging code to new subdirectory

Rearranging the tagging code will make it easier to split the code off
to be a separate library.