#144 Context menu item for setting debugger watch

[Charles Reis]

Story:
User has the debugger open, and would like to watch the
value of a variable. He right clicks on the variable
in the Definitions Pane and selects "Toggle Watch".
The variable is added to the list of watched variables
in the DebugPanel.

Notes:
We don't have any notion for what text is a field or
variable and what isn't, so this can't be foolproof.
But perhaps it can be useful anyway, even if we just
drop the current word (whatever it happens to be) into
the Watches tab. Repeating the action should remove
the variable from the tab.

[Peter Centgraf]

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user_id=431096

This might be easier to implement as a one-way trip. The user
could remove the watch by going to the watch tab. We
should make sure that there is a context menu available there
for removing, since the user will probably expect to keep using
the same interaction style for both functions.

[Charles Reis]

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user_id=429731

Remove Watch is already in the contextual menu for the Watches
table, so we're half-way there.