Do not overwrite Qmax upon loading in a new S(Q) for plot

[marshallmcdonnell]

When we load an S(Q) plot, the GUI restores the Qmax along with other options as well. We should keep current state of these variables and instead have a buttong that says something like 'Restore default settings/options'.

It is more common to find a common Qmax and load multiple S(Q) using the same parameters than to restore them and have to re-type them in every time.

[marshallmcdonnell]

To elaborate and show what we mean about the Qmax options, here is a demonstration of what occurs now:

1. We set a Qmax we want to use for the Fourier Transform to get G(r)
   

2. Then, we load in S(Q) files (the multi-selection feature was added in commit 7ec1a97)
   

3. When we load the files in, we lose the Qmax we set and the G(r) produced use the default Qmax of 31.4 (instead of the Qmax=20)
   

We want to keep the Qmax (and other opitions) set when loading in S(Q) and when we want to restore to the default, have a button to press to do so. Something like 'Restore default setttings'.