When Find Actions is a dialog which has focus, the default
IEclipseContext and derivative ExpressionContext fail at resolving
variable like `activeEditor`.
In CommandProvider, we enforce usage of the right context.
A fix is also necessary for E4HandlerProxy to ensure the right
IEclipseContext is used when one is provided.

Change-Id: I3bbcbe78a709c11135a405121586b660d133211b
Signed-off-by: Mickael Istria <mistria@redhat.com>