Support WINRATE property

[kevinsung]

Part of #487 .

This lets Sabaki recognize when an SGF file has the non-standard WINRATE property attached to move nodes, in which case it would display a winrate graph even with no engine analysis session running. A winrate value from an engine analysis session would override the value from the SGF file.

Here I assume WINRATE represents the probability of black winning as afloat in [0, 1], but I can change this.

[apetresc]

Are these the same non-standard property names that things like goreviewpartner emits in its .rsgf files? If we're going to adopt non-standard extensions, they should at least be de facto standards.

Nevertheless, your contributions are very much appreciated @kevinsung :)

[kevinsung]

@apetresc I have not tried goreviewpartner or any other tool that adds properties to SGF files, though I did read #487 (comment) by the main developer of goreviewpartner. It wasn't clear to me what the "BWWR" property mentioned there meant, so I didn't copy it. I agree that we should try to stick to de facto standards, so I understand that more discussion on these standards may be needed before merging this PR. Perhaps we should continue the discussion over at the linked issue.