UI polish: menus, Esc, section box button, view cube and narrow windows #16

Small interaction issues found in review of #8:

• A hotkey pressed while the context menu is open runs but leaves the menu open and out of date.
• Esc during the viewpoint tour stops the tour and also clears the selection or the measurement (one Esc should do one thing).
• With box mode already active, the toolbar Box button refits the box and discards a hand-adjusted box with no undo.
• Right-clicking the view cube opens the browser's own menu; the cube's arrow and home buttons keep keyboard focus after use; hover highlight on the cube goes stale when it turns under a still pointer.
• Moving "up" from the top view swings 180° around the pole instead of rolling over the edge.
• At about 1024 px width the toolbar overflows, and the truncation banner can cover the panel close button and the cube's home button.
• Disabled context-menu items have no tooltip; counts in the status bar and tree are not locale-formatted.
• A section-plane ring seen edge-on can rotate in jumps.