Blow-up pane option

- Simpler solution to "whiteboard" annotations: Like shutter, select an
area of the screen to create a blowup (creates and saves a png image).
This allows for parts of the presentation to be focused on, and
annotated, and the annotations to be "returnable", e.g. one important
part of code or a website could be blown up and then diagrammed.

**Subtasks:**
- [ ] Blowups can be created (selecting screen area)
- [ ] Blowups are stored in a top-level "blowups" editor (at the same level as slides)
- [ ] A slide-editor like interface can drop in from the right to select blow ups
- [ ] They can be viewed and drawn on (1 simple marker for now)

```
[blowup]
background = _wb_blowups/01_fg.png
foreground = _wb_blowups/01_bg.png
```

**More notes:**

- Route 1: Have "side deck" of slides
- alternate = true
- Blow-up tool automatically makes a side slide that contains 1 pane:
- Drawable image pane
- image-canvas = ./blowups/whatever.png
- Route 2: Make all slides drawable

Start by resolving whether the implementation should use the side-deck approach or make all slides drawable. Work through the four listed subtasks: creating and storing selected-screen blowups, providing the editor interface, and supporting viewing with a marker; completion means each subtask and the documented blowup representation are supported.

Written by the indexing model from the issue text.