spectate.lua: alternative way to make config public

[TymurGubayev]

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[TymurGubayev]

another alternative: public get_config() and get_save_state()

[myk002]

I refactored gui/spectate to do read-only lookups of plugin state and to use the public API for setting the values. That way the cpp part of the plugin stays synced with the Lua

[TymurGubayev]

I see you are now doing dfhack.run_command('spectate', 'set', cfg_elem, text). But parse_commandline is already public and would have the same effect(*). Using it would avoid one layer of indirection.

Also, to be really really sure, we should read the config state on the UI side after dfhack.run_command/parse_commandline is executed, in case it is rejected for whatever reason, and update the UI accordingly.

That said, this is all not important. Please feel free to close this PR.

(*) if there are exceptions depends on what dfhack.run_command does, but that could be solved by adding a pcall on the UI script side.