Closes #1. Closes #2. Closes Trello Cards 18, 23, 24, 25.

Selection start and end selectors now presented as “select” control.

This means, among other things, that Card 18’s task of implementing error handling for Selector edit boxes has been obsoleted; selections presented to the user are guaranteed to be valid.

Ticks another box in the list on Card 14.

RSpec status:	 		17 example, 0 failures, 1 pending
Teaspoon status:	 	125 tests: 0 failures, 0 errors, 1 pending, 0 omitted
Custom RuboCop rules:	None
Inline RuboCop disabling:	None (in non-generated code)

Trello cards authorising work:
* https://trello.com/c/iweMpez5/23-change-selector-edit-boxes-to-list-controls
* https://trello.com/c/ZbFw6WNN/18-error-handling-for-selector-edit-box
* https://trello.com/c/NjdZd75Q/25-close-github-issue-1-element-highlighting
* https://trello.com/c/4fBSsNLw/24-close-github-issue-2-cannot-chain-calls-to-initselectorselectcontrol-setup-method-with-different-selectors
* https://trello.com/c/h6Kw6FkX/14-main-app-landing-page (partial)