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.
Fix format specifier in GIO C++ wrapper string
Set maximum allowed GLib version to 2.32

This corresponds with the version required by GTK+ 3.4, so that API
introduced in post-2.32 releases will emit warnings when used.
Disable writing of ID3v1 tags by default

ID3v1 tags have many problems, including the lack of a defined character
encoding (and a western-centric ISO-8859-1 convention), 30 bytes per
field and an extremely limited set of fields. Depending on the behaviour
of media players, reading the ID3v1 tag instead of the ID3v2 tag could
give misleading information (truncated fields, mojibake and so on).

As ID3v2 is in wide use, and solves the above problems, disable ID3v1 by
default.

https://mail.gnome.org/archives/easytag-list/2014-August/msg00017.html
Enable ID3v2.3 (instead of ID3v2.4) by default

Although ID3v2.4 is nearly as old as ID3v2.3, it has not received the
same level of support in hardware and software players until recently.
To make sure that users are not confused by their valid ID3v2.4 tags not
being shown in players which only support ID3v2.3, write ID3v2.3 tags by
default.

https://mail.gnome.org/archives/easytag-list/2014-August/msg00014.html
Split progress bar from status bar

There is no need to subclass GtkProgressBar, so just move the progress
bar code to a separate file.
Move status bar to EtStatusBar object
Add "music" to the desktop file keywords
Review strings
Move scanner preferences page to GtkBuilder
Move confirmation preferences page to GtkBuilder
Move UI and misc preferences pages to GtkBuilder

Unify the UI settings in a single User Interface page.
Move file preferences page to GtkBuilder
Move tags preferences page to GtkBuilder
Move ID3 preferences page to GtkBuilder
Move CDDB preferences page to GtkBuilder
Move preferences notebook into GtkBuilder