Fix broken InputAlert

[Teknomancer]

Lots of changes to Haiku's BAlert implementation (using LayoutKit, changing view names etc.)

Currently using InputAlert causes a hard crash, this needs to be fixed.

I already have a working solution but some UI elements aren't perfect (margins), so I'll fix this myself for now.

[Teknomancer]

Addressed the crash and layout related fixes in a1c52e7.

The dialog could be made better with adding some horizontal space between the input alert and the window margins if possible, but that's for later.