move link_aware from being a property of controls to prop of labels

tcontrol now gets it by a virtual method, whose default impl is
false, and is only overrided in label.

This refactor cleans things up, since link aware is really only
relevant to the click handler of label.
make labels read "link_aware" property from their widget defn

This adds a "link_aware" key to label definitions, and C++ tlabel
objects no longer initializer this to true, but instead read the
value from their definition configuration.
add link_color property of label defn, passed to font::ttext

This commit permits link_color to be specified in any label defn,
at the same place that link_aware is specified. It will be passed
through to tcanvas and finally font::ttext in a similar manner as
link awareness, and used for the color parameter in any link
formatting. We choose a default value of #ffff00 everywhere,
matching the definition currently in the help browser for cross-
references.
move default values of link aware, color out of tcontrol header