webui: url decode POST values; fix seeking

In the webui, I noticed the seeking wasn't working correctly, it would always seek to 3 minutes and some seconds, regardless of the video.

I took a look at `CWebClientSocket::OnCommand` in `WebClientSocket.cpp` which handles the seeking and I noticed it was parsing the `position` parameter as if it were already url decoded, but in fact it wasn't.

I changed the parsing to take into account the url encoded colons (`%3A`), it still supports the optional milliseconds parameter. I also did away with the `TCHAR c` variable which was used as a placeholder to parse the colons.

This is my first contribution, so let me know if there's something I should do differently.