This adds a dialog in the Tools menu to open a console and evaluate
Lua code using the embedded Lua engine. It replaces the previous
console.lua implementation that was more limited to use, because
it relies on GUI bits exposed to Lua. It used two separate windows
for that reason.
The implementation uses the existing "funnel" API amd relies heavily
on callbacks to maintain separation between GUI and epan code and
make it generic enough to possibly support more use cases than just
the Lua 5.2 console.
The open and close callbacks are used to install and remove a custom
print() lua function with dialog creation and destruction.
The eval callback is basically the same as luaL_dostring().
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