Introduce focus and shortcut contexts

[kasbah]

This is largely a refactor of focus and keyboard shortcut management (closes #161). It also:

• Implements shortcuts for undo/redo (supersedes Keyboard shortcuts for undo/redo #1186)
• Fixes Keyboard navigation of cells interrupted by completion popup #1256 as best we can without changing completions to not to use up/down: when doing keyboard navigation you might sometimes have to Esc out of a completion popup.
• Fixes Focus seems wrong when opening analysis tab #159
• Adds focus indication to the side-bar: