Full-fledge visual ROI tool

[stscieisenhamer]

Upgrade the ROI tool to have the same functionality as the selection tools of many popular graphics editors. Features would include:

• Visually distinct from the subsets created.
• Show/hide with the toggling of the respective tool mode buttons.
• Visual handles to move, rotate, and resize.
• Action to apply the ROI, such as the creation and modification of subsets.

This is based off a phone conversion with @astrofrog and is becoming more of an issue with the increased functionality being added, as found in #657 and #689.

[astrofrog]

Just for info, @ChrisBeaumont and I started to try and deal with the third point in this package: https://github.com/glue-viz/bermuda but we didn't get very far. But I agree that with draggable subsets, this becomes more important (because otherwise it's hard to tell where the boundaries of ROIs are in e.g. a scatter plot).