advanced feature: allow user to modify the values in the filter menu

[jwist]

make the values in the filter menu editable. For example if a user wants to change the line broadening parameter.

Beware that for some filter not all values are acceptable and may break functions. For example zero-filling only accept powers of two.

[hamed-musallam]

@lpatiny
I remember at the end of discussion of this point we are not agreed on this it?

[lpatiny]

First let do the 'target' that allows to visualise a spectrum at a defined level of filters. This should allow to edit correctly with the right tool the parmeters

[hamed-musallam]

@lpatiny
it's done, can you check if this what you need?