Adds compatibility for latest libui

[emersion]

Do not merge before a new release of libui is out.

• New{Password,Search}Entry
• Form
• Grid (missing InsertAt)
• Window.SetPosition
• Window.Center
• RadioButtons selection support
• MultilineEntry (MultilineEntry added #160)
• Other new Window functions

Contributors: you can make pull requests to https://github.com/ProtonMail/ui/tree/latest-libui

[mainrs]

Is it possible to use this pull request for my go get so I can use the Center option of Window?

[emersion]

If you use Glide, you can just change repo in glide.yml. Otherwise, you can change all your imports from github.com/andlabs/ui to github.com/ProtonMail/ui.

[andlabs]

The fate of uiWindowCenter() is uncertain. (If there's a correct way to write it in GTK+ that works regardless of whether the window was ever shown at all in the past, then it will stay.)