gtksourceview: new package

gtksourceview: new package

GtkSourceView is a portable C library that extends the
standard GTK+ framework for multiline text editing with
support for configurable syntax highlighting, unlimited
undo/redo, search and replace, a completion framework,
printing and other features typical of a source code editor.

https://projects.gnome.org/gtksourceview

Signed-off-by: Fabrice Fontaine <fabrice.fontaine@orange.com>
[Thomas:
 - simplify the Config.in by using "depends on BR2_PACKAGE_LIBGTK3"
   instead of a select. It is pretty obvious that Gtk is needed if you
   want to use gtksourceview.
 - license is LGPLv2.1+, not LGPLv2+.
 - change location in package/Config.in, gtksourceview is a library,
   it should not go under "Development tools".]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>