Add condition HelpSystem.getContext(contextId) != null to check if
marker icon should be annotated with question mark.

The check is the same as in
org.eclipse.ui.internal.help.WorkbenchHelpSystem:displayHelp(String)
So the problem view now does not show the help icon exactly when no help
would open.

Introduced a new (hidden and enabled by default) preference to control
this new help check behavior. This allows product intagrators disable
the help check if they use RemoteContextProvider and enabled RemoteHelp
via org.eclipse.help.base/remoteHelpOn=true.

To disable the new preference, product customization file should have
this line:

org.eclipse.ui.ide/helpContextAvailabilityCheck=false

Change-Id: Ic102ebeefbcb5a36d55022d3c1994dc589d83c88
Signed-off-by: Tim Neumann <Tim.Neumann@advantest.com>