Make sure we don't call back into GtkEntryBuffer

Make sure we don't call back into GtkEntryBuffer

After commit 7e77afe94c14365d538c958d23c1e3e074530415 moved the deletion
of text into the signal handler, in order to make undo work, we need to
override the GtkEntryBuffer::deleted-text class closure when subclassing
GtkEntryBuffer, as well as overriding GtkEntryBufferClass.delete_text,
otherwise the default class closure will be invoked, and will try to
delete an empty buffer.

Fixes: #3140