ENH: use icons for markups visibility

Change from a text box in the table to an open or closed eye icon.
Change that set of item widgets to have a user role of true/false
rather than using the check state.
Use the visible/invisible icon in the header - todo: fix scaling issue.
ENH: use lock/unlock icon for lock column

Use an icon for the lock column header, as well as for
the selected column header, adding tool tips.
ENH: add some list wide functionality

Added buttons, signals, slots, methods to operate on all markups
in a markups node. Toggling visibility, selected, locked, as well
as moving markups up/down in the list.
Added tests for new functionality.

To do: debug deleting