Add imgui_ctx.tree_node_ex (flags are missing in imgui_ctx.tree_node)

[zaicruvoir1rominet]

Hi there !

Adding tree_node_ex to imgui_ctx, which would allow the following:

from imgui_bundle import imgui, imgui_ctx

with imgui_ctx.tree_node_ex("test", imgui.TreeNodeFlags_.default_open.value) as tree_node:
    if tree_node.visible:
        imgui.text('imgui_bundle is awsome, thanks')

/!\ Warnings:

• I added the code manually, I don't know if imgui_ctx is supposed to be auto-generated or not
• I tested this change on a side project, but didn't see any test plan for imgui_ctx in this repo, so do tell if there's a specific location where I should ask the tests

[pthom]

Hi @zaicruvoir1rominet

Thanks a lot for this very nice contribution! I just merged it.

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Ps: As a french guy, I love your username !