Add CollapsingHeaderV with flags parameter

[ptxmac]

This allows TreeNode flags to be passed along. In particular it's useful for having headers default to open:

if imgui.CollapsingHeaderV("Header", imgui.TreeNodeFlagsDefaultOpen) {
  // ....
}

[dertseha]

Hello there and thank you for your contribution.
If you could add a minor change please, then it's good to go.

[dertseha]

Thank you!