Add/expose isVisibile, show, and hide methods for SidebarView object

For Issue #1187 

Added public API functions for SidebarView object: show(), hide(), and visible().

Changed public API function: toggleSidebar() to toggle().

Did a find in files for current Brackets codebase and all currently listed Brackets extensions.  toggleSidebar() is not used anywhere except for locally in SidebarView.js.

Ran unit tests without any problems.  I'm not sure that SidebarView has any unit tests right now.  If you want me to research and possibly create unit tests I can.  Otherwise, this fix is complete and ready for review.