soffice: Do not announce selection changes from unfocused combo boxes

See merge request !34

This is useful in LibreOffice 4.2 as the style toolbar updates when
navigating the document, and its combo boxes emit the signals when they
display the new value, leading to noise.

Migrate from intltool to gettext

See merge request !33

Recent gettext version can extract and merge back strings from and to
various file formats, no need for  intltool anymore.

https://wiki.gnome.org/Initiatives/GnomeGoals/GettextMigration

#35

(cherry picked from commit 7ed7c80c)

Note that there will be no difference in behavior until toolkits add
support for this new functionality.

This is to work around Firefox not reliably updating the caret position when
navigating by heading.