Issues-view menu improvements & Variable-based issue context resolution

This adds a new [Resource] valued property for Issue instances that is
by default defined to just read the old Issue.HasContexts L0.List
property. The property allows for more specific domains to override the
issue context resource calculations.

To support this calculation a noteworthy change needed to be made to
org.simantics.db.layer0.function.All.getStandardChildDomainPropertyVariable.
It now performs a new final fallback step that resembles how the
standard procedural variables work. If the parent variable has a solver
variable node defined, it will use NodeManager.getClassifications as an
attempt to read the type Resource of the variable. If this is
successful, the asserted properties of the type are searched to find a
property matching name of the the requested property. To support this a
new request was added: UnescapedAssertedPropertyMapOfResource. This
request is also used to optimize the implementation of
All.standardGetValue[12].

These changes also add a Help action for Issues view context menu that
looks for the "contextHelpId" String property from the Variable
describing the issue.

refs #6948

Change-Id: I9655ea3647851fa04fb2420686cb13dd0c6719b6