Specialize the call{ers,ees,graph} queries into the UI

[fzipp]

Specialize the call{ers,ees,graph} queries into the UI: fetch the complete callgraph at startup and cache it. Use it to mark up the source so that all function declarations have a little "jump to caller(s)" button in the margin that performs the "callers" query (from the cached callgraph), and all call sites have a hyperlink on the '(' token that performs a "callees" query (also from the cache). If the result is unique, go straight there without showing the results pane.