refactor(document): Firefox uses its own key-event handlers

Preparation for modifying key-events handlers to work with Firefox.
WIP(document): caret moves within BoldText node

Exploring key-events handling strategies.
WIP(document): selenium tests are more clearly structured

Selenium is initialized and closed at the module level.

Code manipulating django and selenium is now a separate class.

CaretInParagraphTests is broken down into a base, with testing code,
and smaller subclasses, with parts of the case data.
WIP(document): first insertion tests
Merge branch 'firefox-support' of https://github.com/maciej-filip-sz/fiduswriter into selenium