Add a way to unselect current tech goal

[daavko]

Is your feature request related to a problem? Please describe. One may not always want a tech goal, and there's no good way to unselect it after you've selected one.

Describe the solution you'd like Some way, perhaps a button on the research screen, to unselect the tech goal you've chosen.

Describe alternatives you've considered Leaving it as-is, which would make some people very sad.

Additional context This already made some people sad in LT76.