epic: menus, tables, trees and value editors #85

Goal

The widgets a desktop-grade tool expects that blit does not have yet. Each is its own sub-issue and its own PR.

Sub-Issues

• feat: menus, menu bars and context menus #104 menus, menu bars and context menus
• feat: tab bars #105 tab bars
• feat: a tree view #106 a tree view
• feat: tables #107 tables
• feat: value editors and a colour picker #108 value editors and a colour picker
• feat: radio buttons, progress bars, separators and a filtering combo #109 radio buttons, progress bars, separators and a filtering combo
• feat: richer lists #110 richer lists

Sequencing

All need hashed ids (#94), and most need the state store (#95). Once those land they run in parallel, except #105, which waits for drag and drop (#99), and #108's colour picker, which waits for the vector primitives (#79). #83 (docking) reuses #105.