895 export base pair lines

[rayzhuca]

Added export SVG support for base pair lines.

Changes

• Removed the old method of creating a separate method to filter selected base pairs as suggested.
• Modified _base_pairs(bool allow_mismatches) to _base_pairs(bool allow_mismatches, BuiltSet<Strand> selected_strands). Now base_pairs is this._base_pairs(false, strands.toBuiltSet()) and selected_base_pairs(BuiltSet<Strand> selected_strands) just calls _base_pairs directly.
• For each base pair, I added an id base_pair-{helix-id}-{offset} so we can identify the base pairs that is in selected_base_pairs.
• Created a new method List<Element> get_selected_base_pairs(Store<AppState> store) that mirrors the behavior of List<Element> get_selected_strands(Store<AppState> store) so that it adds the base pair HTML elements into the exported SVG.

[rayzhuca]

It should work now. It was a quick fix. In export_svg_middleware, there is an if statement. One branch is for ExportSvg and another is CopySelectedStandsToClipboardImage. I just had to call get_selected_base_pairs in both branches.

[rayzhuca]

The new commit renamed get_selected_strands and get_selected_base_pairs to get_svg_elements_of_strands and get_svg_elements_of_base_pairs as well as changes to function arguments. I also removed the long one-line and refactored two pieces of repeated of code into one function called get_selected_svg_elements.