Modal full-screen source viewer

The source view (#34) in the context of the explorer (#63) has an "enlarge" button. Clicking this button should lead to a rearrangement of the elements on the screen, such that:

- [ ] The source text takes up all of the height and most of the width of the window, although the number of characters on a line should be kept within a reasonable limit (100?) in order to maintain readability. Perhaps the font size could be enlarged a bit to compensate for an overabundance of horizontal space.
- [ ] Panels that used to be on the left and right of the source view disappear out of view "behind" the edges of the screen. They can be reopened as "drawers" by clicking on small tabs at the edges of the screen. When a drawer is opened, it temporarily covers part of the source text rather than the text moving out of the way.
- [ ] All functionality continues to work as it would in the unenlarged scenario. Highlights, x-ray mode, edit mode etcetera can be toggled on and off.
- [ ] Highlights are still aligned with the source text. This might or might not require recalculating their positions.

In order to implement this, it's probably most straightforward to create a new view which is placed as a modal in front of the explorer and which "borrows" the explorer's panels, repositioning and potentially rescaling them. This modal view could either be owned by the explorer, or it could be a global that is directly controlled by the search and exploration aspect (#106).

Some details need further investigation. For example, when opening the annotations drawer on the right (#39), it is arguably desirable to be able to click on an annotation summary block, see the annotation's details (#45) and then click through to related items (#46), all without leaving the full-screen view. The question is then how to display that sequence of panels in the context of a drawer.

This feature interacts in interesting ways with several other issues: #43, #62, #106 (list not exhaustive).