Redesign curve fitting dialog

[sstendahl]

Redesigns the curve fitting dialog, addresses the points in the GNOME Circle feedback, and makes the dialog more responsive for different screen sizes.

Before:

After:

[cmkohnen]

The "Graphs (Development)" Title on this feels off.
Also since this is kind of a modal dialog, I don't know if this style of sidebar would fit like it does in the main application

[sstendahl]

The "Graphs" title on the left probably feels better for a stable build, so without the "(Development)" part. But I think I agree with you that the Splitview look is probably a bit weird with a modal window. It kinda feels like a second application that's stuck to the main window, instead of it being a modal dialog.

The other issue, that did annoy me somewhat, is that the Action buttons simply clash with this design. If you resize the window the cancel button will be in the sidebar. So in compact mode only the graph is shown. To then dismiss the window, you have to expand the sidebar first to reveal the cancel button. It just feels off. Adding the cancel button to the content view in compact mode works but doesn't feel ideal either. So if we'd go with a Splitview, we'd probably have to change the controlls a bit.

I updated it to be more in line with regular modal windows, so closer to the orginal. See screenshot:

[sstendahl]

Made some more tweaks, this now also disables interaction with the curve fitting window graph and adds the labels back to the graph.

[sstendahl]

For reference, here's with the updated figure style: