qt: Collapsible sections for Mempool and Spam filtering tabs

[kwsantiago]

Closes #71

Group options into collapsible sections and wrap Mempool tab in a scroll area.

[luke-jr]

Checkboxes suggest the user is changing settings, and the wasted blank space for a collapsed section seems to defeat the point...

Maybe something looking like this: https://stackoverflow.com/questions/32476006/how-to-make-an-expandable-collapsable-section-widget-in-qt

[luke-jr]

Maybe open collapsed, but with Expand/Collapse All buttons?

eg https://ux.stackexchange.com/questions/138468/expand-all-collapse-all-expand-all-of-type-collapse-all-of-type

I also feel like the lines to the right of the section label should be clickable

[luke-jr]

Thoughts on rearranging the furniture a bit at the same time? (In a separate commit)

7924862

[luke-jr]

I can click the line now, but ONLY the line (quite thin) - probably need to detect clicks on the full height of the text.

There's also a custom cursor for the line, but not for the text. (Both should match whatever the OS normally uses for these kind of widgets)

Finally, because things are collapsed by default, the ModScrollArea starts with a smaller width, and upon expanding grows a horizontal scrollbar, which is ugly.