Avoid a potentially-uninitialised variable

https://bugzilla.gnome.org/show_bug.cgi?id=727295
Switch to use GTK+ 3 by default
Show progress bar text with GTK+ 3
Support reading tags from Ogg Opus files

https://bugzilla.gnome.org/show_bug.cgi?id=692389
Support for writing tags to Ogg Opus files

https://bugzilla.gnome.org/show_bug.cgi?id=692389
Fix reading header information from Opus files

Correctly read the sample rate and duration from Opus files.
Additionally, handle images in an Opus comment.
Update AppData for Opus file support
Error out if Ogg disabled but Speex/Opus enabled

Opus and Speex support depend on Ogg support for parsing of Vorbis
comments, used in both formats.
Rearrange Opus/Speex Ogg checks

Place the checks inside the path where Opus/Speex support is enabled, to
prevent superfluous errors.
Updated Brazilian Portuguese translation
Mention Ogg Speex support in AppData description
Update British English translation
Updated Polish translation
Updated Brazilian Portuguese translation
Small optimization for scanner functions

Discovered while fixing bug 726108.
Updated Spanish translation
Reimplement Roman numeral detection

Roman numeral detection code used complicated process of check, parse,
build, compare to detect if a string was a valid Roman numeral.
Reimplemented as a single step, that only checks for the validity (and
well-formedness) of the string, without computing its value.

It uses the same modern Roman numeral notation, including subtractive
notation.
Update about dialogue credits
Updated German translation
Updated Czech translation