Added description to `plusIconVisible` property.

Using more modern (dot) syntax in implementation.

Drawing plus button in corner.

Only drawing plus icon when `plusIconVisible` is set to `YES`.

Drawing plus icon in correct fill color depending on enabled/highlighted states.

Only changing (and consequently redrawing) `highlighted` when necessary.

Marking button as needing to be redrawn when changing plus icon visible property.

Fixed an issue where rotation was allowed and caused buttons to appear out of visible view.

Cleaning up main view controller.

Converting tabs to spaces.