Modify "Actions" menu in the Assembly Browser

Currently, the menu contains only a few items (see the screenshot). The menu will contain all main actions. For now, restructure the existent items and add "Remove reference sequence" item.

Thus, the menu will look as follows:

• Assembly
  • Export assembly to SAM format
  • Export assembly region
• Reference
  • Set reference sequence
  • Remove reference sequence
• Appearance
  • Zoom in
  • Zoom out
  • -----------------
  • Export as image
• ------------------
• Close active view

For the "Remove reference sequence" item use the attached icon. The item should be disabled if the reference sequence is not set. Also rename the "Unassociate" item in the Reference Area context menu to "Remove reference sequence".