Add "Show atoms" graphics window context menu

[goddard@…]

The following bug report has been submitted:
Platform:        Windows-10-10.0.17763
ChimeraX Version: 0.92 (2020-02-13)
Description
ctrl-double-click on ribbon context menu has "Hide atoms" menu entry.  Would be nice if it had "Show atoms".

Log:
UCSF ChimeraX version: 0.92 (2020-02-13)  
© 2016-2020 Regents of the University of California. All rights reserved.  
How to cite UCSF ChimeraX  

> open 6pt2 format mmCIF fromDatabase pdb

6pt2 title:  
Crystal structure of the active δ opioid receptor in complex with the peptide
agonist KGCHM07 [more info...]  
  
Chain information for 6pt2 #1  
---  
Chain | Description  
A B | δ opioid receptor  
C D | Peptide agonist KGCHM07  
  
Non-standard residues in 6pt2 #1  
---  
CLR — cholesterol  
OLA — oleic acid  
OLC — (2R)-2,3-dihydroxypropyl (9Z)-octadec-9-enoate (1-Oleoyl-R-glycerol)  
OXJ — 1-[3,5-bis(trifluoromethyl)phenyl]-N-methylmethanamine  
  
  

> select /A:1055

6 atoms, 5 bonds, 1 model selected  




OpenGL version: 3.3.0 NVIDIA 430.39
OpenGL renderer: Quadro P6000/PCIe/SSE2
OpenGL vendor: NVIDIA Corporation