Improved answer editing for parentheses, grouped expressions, and variables such as dt and dx. Fixed editor dismissal and recovery issues, and added a notification bell with improved notification controls.

Psetter 0.1.3 adds Firefox support and improves MITx answer compatibility.

• Fixed editing and round-tripping of existing MITx answers.
• Added support for physics vector aliases: hati, hatj, and hatk.
• Improved handling of Greek symbols and Latin variables such as o.
• Added queued developer notifications, allowing multiple notices to be shown one at a time.
• Added target-specific Chrome and Firefox Store builds.