Introducing 'Peek Definition' commands

[retailcoder]

Adds a Peek Definition command to the Code Explorer, code pane, and VBE Project Explorer context menus, and introduces a method in ISelectedDeclarationProvider to get the SelectedProjectExplorerModule; derived a PE-specific Find all References command from the existing one, overriding OnExecute to explicitly get the its target from the PE. This fixes a glitch with PE navigation commands where the command would acquire its target from the active code pane.

Opportunistically, also fixes a bug that looks like it went under the radar: derived commands' SpecialEvaluateCanExecute logic wouldn't be invoked because the method wasn't virtual and merely being shadowed, causing only the base method to run (and systematically return true). This PR makes the derived methods proper override that do get invoked. edit; nope, this was actually a misguided refactoring that was undone in the PeekDefinitionCommand but forgotten in the other ones

[bclothier]

Love the idea! I do not think we should be changing the can execute conditions to a virtual method, however because we do want to preserve the hierarchy of parent's condition before checking the conditions required by the child. virtual destroys that hierarchy.

[bclothier]

I have some concerns about the protected usages. I think those should be simply private as well. I'm fine with renaming of EvaluateCanExecute but as a note, I believe the reason SpecialEvaluateCanExecute was used to avoid confusion with some EvalulateCanExecute method that already exists in the base.

[retailcoder]

The ESC input binding isn't working, it may need to go on the actual parent Popup control. Will fool around and find out =)

done

[retailcoder]

Unless there are further review comments, this feature is ready to ship.