Simplify tui_data_window::show_register_group

This simplifies tui_data_window::show_register_group, renaming it as
well.  The old method had two loops to iterate over all the registers
for the arch, but in the new scheme, the vector is set up when
switching groups, and then updates simply iterate over the vector.

tui_data_item_window is made self-updating, which also clarifies the
code somewhat.

Tested-By: Tom de Vries <tdevries@suse.de>
Reviewed-By: Andrew Burgess <aburgess@redhat.com>
Approved-By: Andrew Burgess <aburgess@redhat.com>