3D Structure View action on global sequence toolbar

Currently, the 3D structure view exists almost independently from the sequence view: you can open a 3D model only from the project view or from the 3D structure view toolbar.

The basic issue: add a 3D structure view button to the toolbar as it is done for the Circular View.

Steps to fix the issue:

• Add a button with the 3D view structure icon to the global toolbar near the Toggle circular views button.
  • This button is enabled, if file have a 3D structure and it is disabled in other cases.
  • Name/tooltip of the button should be Toggle 3D structure views
• Local toolbar should have height as height of the sequence view local toolbar.
• Rename the local toolbar header "3D Structure Viewer" to "3D Structure View".
• Add the Close button with the '|' delimiter to the local toolbar of the 3D structure view, as it is done on the sequence view local toolbar.