ui: Port main window to gtk+3

Makes the main window load
ui: Fix build

Signed-off-by: Germán Poo-Caamaño <gpoo@gnome.org>
document: Port about dialog to gtk+3

Signed-off-by: Germán Poo-Caamaño <gpoo@gnome.org>
utils: Port GladeWindow class to GtkBuilder

Signed-off-by: Germán Poo-Caamaño <gpoo@gnome.org>
format: Port dialog to gtk+3

Regression: It is necessary to change the widget to choose
the output file

Signed-off-by: Germán Poo-Caamaño <gpoo@gnome.org>
sort: Port dialog to gtk+3

Signed-off-by: Germán Poo-Caamaño <gpoo@gnome.org>
sort: Move 'set default button' to the action area

Group buttons in one single action area

Signed-off-by: Germán Poo-Caamaño <gpoo@gnome.org>
openurl: Port dialog to gtk+3

Signed-off-by: Germán Poo-Caamaño <gpoo@gnome.org>
search: Port dialog to gtk+3

Signed-off-by: Germán Poo-Caamaño <gpoo@gnome.org>
document: Add ellipsis to menus that open dialogs

Signed-off-by: Germán Poo-Caamaño <gpoo@gnome.org>
editor: Port to gtk+3

Signed-off-by: Germán Poo-Caamaño <gpoo@gnome.org>
editor: Add padding to automatic fields
preferences: Port Config dialog to gtk+3
fields: Port dialog to gtk+3
ui: Remove unused gladep files
editor: Port entry type change in editor to Gtk3
index: Fix call to popup menu
document: Remove comment

Statusbar works in Gtk3 port.
utils: Port set_cursor to Gtk3
editor: Port create field to gtk+3