Add call to assignCtrlIDs in TabWidget.onInsertedPage()

[merlinz01]

See #107 (comment) for more.

[dblohm7]

Would you mind authoring and submitting a PR that resolves this?

Tailscalars generally author PRs to this repo for issues directly affecting our GUI; since we don't use TabWidget or TabPage, we aren't going to be able to spend time on this. Having said that, I'll gladly review and merge a PR!

[merlinz01]

I'll see if I can find time to do so.

This repo is a fork of github.com/lxn/walk, which seems to be unmaintained for several years. Is Tailscale intending to maintain this fork indefinitely apart from the original repo?