Add "Copy", "Paste", and "Delete" to the r-click drop down menu in sequence mode

[ljubica-milovic]

Background
This ticket is a complementary one to the ticket from the previous release: #6252.

Requirements

1. Options "Copy", "Paste" and "Delete" should be added to the r-click drop down menu:

1.1. When monomers are selected "Copy" will copy the selected monomers, "Paste" will replace them by the pasted monomers, and "Delete" will delete them.

1.2. R-clicking on a sequence when no monomers are selected "Copy", "Paste", and "Delete" should be disabled.

1.3. When r-clicking outside of the sequence "Copy" and "Delete" should be disabled, and "Paste" enabled.

Options "Establish Hydrogen Bonds" and "Delete Hydrogen Bonds" should also be added - covered in the scope of #6254 (requirements 1).

UX