Fix several bugs with diagram tool modes.

Fix several bugs with diagram tool modes.

- Remove the auxiliary creation mode from the tool bar as it was not implemented in the first place
- Fix a NullPointerException in flow creation mode which occurred when the mouse pointer hovered over diagram elements
- Make variable creation shortcuts (shift-a, shift-s etc.) to only work in the basic pointer mode, this also fixes a bug which allowed variables to be created when the diagram was in the lock mode
- Clean up DiagramToolHandler, context was initialized inside a separate thread even though there does not seem to be any reason to do so. Revert this change if something breaks.

fixes #5758

git-svn-id: https://www.simantics.org/svn/simantics/sysdyn/trunk@31159 ac1ea38d-2e2b-0410-8846-a27921b304fc