Fix GtkEntry drawing to provide visual feedback when editable=FALSE.

GtkEntry currently draws exactly the same no matter what the state of the
'editable' property. This is pretty confusing for users because there's
no visual feedback at all, it just seems like their keyboard is broken.

This change adds a "read-only" class to the StyleContext, which will
continue to allow the user to select/copy the text, but will draw the
entry as if it were insensitive, providing some indication that the
contents can't be changed.

Signed-off-by: David Trowbridge <trowbrds@gmail.com>

http://bugzilla.gnome.org/show_bug.cgi?id=694831
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
Fix a typo
Revert "Add the "text-button" style to button created with gtk_button_new_with_{label,mnemonic}"

This reverts commit 8d7bab7d7b902e54f27d7dbe26bca9e21eb45787.
Redo style class handling

We should set the appropriate style classes when we have
constructed the content and know if it is a label, an image,
or both. Doing this in the convenience constructors is
problematic for language bindings, and misses out when the
content is changed after construction.