Add "Undock tool" menu entry

[goddard@…]

The following bug report has been submitted:
Platform:        Darwin-19.3.0-x86_64-i386-64bit
ChimeraX Version: 0.92 (2020-01-31)
Description
Would be nice if toolbar and command line had an "Undock" context menu entry.  Usually when I have undocked these tools I have wanted to be able to redock them, and also I don't want the undocked setting to be remembered, and that is what this proposed "Undock" menu entry would do.  The current "Dockable Tool" checkbutton menu entry is also useful and should also be in the menu.

Log:
Startup Messages  
---  
warning | 'clip' is a prefix of an existing command 'clipper'  
  
UCSF ChimeraX version: 0.92 (2020-01-31)  
© 2016-2020 Regents of the University of California. All rights reserved.  
How to cite UCSF ChimeraX  

> open 6ack format mmCIF fromDatabase pdb

6ack title:  
Trypsin-cleaved and low pH-treated SARS-CoV spike glycoprotein and ACE2
complex, ACE2-bound conformation 3 [more info...]  
  
Chain information for 6ack #1  
---  
Chain | Description  
A B C | Spike glycoprotein  
D | Angiotensin-converting enzyme 2  
  
  




OpenGL version: 4.1 ATI-3.5.5
OpenGL renderer: AMD Radeon Pro 580 OpenGL Engine
OpenGL vendor: ATI Technologies Inc.

[Tom Goddard]

Not a high priority, but didn't want to forget this idea.

[pett]

Menu item added for tools with no title bars