GtkEntry: add a "tabs" property for setting a PangoTabArray in the layout.

GtkEntry: add a "tabs" property for setting a PangoTabArray in the layout.

I'm currently working on porting view::FieldEntry (from libview) to C for use in
upstream GTK+. FieldEntry is a widget which allows users to enter structured
text such as IPv4 addresses or serial numbers. The way that FieldEntry
delineates the fields within the entry is with tabstops, using PangoTabArray
entries to precisely position the fields and delimiters. Because GtkEntry
rebuilds its internal PangoLayout fairly frequently, this requires a property in
the entry that will set the tabs on the layout whenever that happens. This API
looks very similar to one in GtkTextView.

Patch by David Trowbridge <trowbrds@gmail.com>. Updated for Gtk+ 3.10.

https://bugzilla.gnome.org/show_bug.cgi?id=697399