Create API for manipulating sequence feature display on AlignPanel

Sequence feature display and rendering style code in Jalview 2 is tightly linked to the GUI implementation classes, and a number of different calls are needed to programmatically enable/disable the display of sequence features and ensure all GUI elements (checkboxes in menus and view panels) are updated to reflect change in an AlignmentView's settings.

The new API should, in the first instance, provide simple calls to control feature overlay and individual feature/group display, and set/get feature styles.