KEY-2 Navigate between highlights

[Nitrooos]

For end user:

• navigating between highlights using arrow keys
• possibility to simulate 'click' event on given element by pressing Enter when proper highlight is selected

Technical summary:

• remove highlightsModule, now showing/hiding highlights is the responsibility of domHighlightModule (it's the only one making operations on DOM)
• create appModule, encapsulating all state and high-level operations, like 'toggleHighlights', 'navigateHighlights' and 'simulateClick'
• created utilsModule with some very basic util functions
• assign additional data to highlight objects: bounding client rect, original clickable element and the actual highlight element itself