New way of adding new regex model strings, and editing the text of existing ones
Render the regular expression with operator characters emboldened
Handle the emboldening of the backslash-prefixed letter operators in the regex text field
Refactor main view controller into two, one embedded in the other, for better separation of concerns and improved ability to let UIKit do more keyboard-related fit-and-finish, like moving the table around so that text fields are not obscured by the keyboard sliding up.
Added accessory keyboard as inputView of the regular expression editing field.