Bump minimum gtk+ version to 3.11.5

... and remove conditional compilation for older gtk+ versions.
Set the main window as the parent for error dialogs

https://bugzilla.gnome.org/show_bug.cgi?id=725579
Break out the history dialog as a separate object

At the same time, move to a separate GtkBuilder template.

https://bugzilla.gnome.org/show_bug.cgi?id=725579
Break out the update dialog as a separate object

At the same time, move to a separate GtkBuilder template.

https://bugzilla.gnome.org/show_bug.cgi?id=725579
Updated Spanish translation