Rhiju/graphicsspeedup roundedrectproteins

[rhiju]

Addressing issues #18, #21, #22

• [x] Delete graphics objects when not in view (rather than setting their 'visible' properties to false)
• [x] Allow hiding of noncanonical_pair and stem_pair for super-streamlined view.
• [x] Simpler "rounded rectangle" representation for proteins.
• [x] Allow global toggling of selection labels -- draw_selection becomes rate-limiting and very slow for 16S, 23S, etc. when linkers are off.
• [x] Bug fixes: stack linkers were called base_stack; tertiary_contact & long_range_stem_pair confusion.