Filter Wizard: make save to coefficients button work like apply filter design

In short, make it a push button that's always visible but disabled when
not relevant.

Signed-off-by: Tim Harder <timothy.harder@analog.com>
Filter Wizard: disable save2workspace button on filter type change

Signed-off-by: Tim Harder <timothy.harder@analog.com>
Filter Wizard: relabel save2workspace button

Usage from matlab can change the string via a passed value to something
more relevant such as "Apply filter design."

Signed-off-by: Tim Harder <timothy.harder@analog.com>