UI: Add function to find default video save path

On windows this will return the documents\video directory, but on
linux/mac it'll just return $HOME for the time being because I don't
know if there really are any other appropriate adequate paths to use.
Perhaps someone else can be willing to fill this in if they wish.

obs/platform-x11.cpp

@@ -101,3 +101,8 @@ bool InitApplicationBundle()
 {
 	return true;
 }
+
+string GetDefaultVideoSavePath()
+{
+	return string(getenv("HOME"));
+}