Added the initial planning document for the portable API.

plan.md

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+# Go UI package planning
+Pietro Gagliardi
+http://github.com/andlabs
+
+## Goals
+- Simple, easy to use GUI library for hard-coding GUI layouts
+- Go-like: uses Go's concurrency features, interfaces, etc. and behaves like other Go libraries
+- Portable; runs on all OSs Go supports and uses native toolkits (wherever possible)
+- Minimal: only support what's absolutely necessary (for instance, only events that we will actually use in a program); if functionality can be done cleanly in an existing thing, use that (for instnaces, if adjustable sliding dividers are ever added, they can be made part of `Stack` instead of their own thing)
+- Lightweight and fast
+- Error-safe
+- Correct: uses APIs properly and conforms to system-specific UI design guidelines
+
+## Layouts
+Layouts control positioning and sizing. Layouts are controls, so they can be added recursively. The layout types are:
+* `Stack`: a stack of controls, all sized alike, with padding between controls and spacing around the whole set. Controls can be arranged horizontally or vertically. (Analogues: Qt's `QBoxLayout`)
+>* TODO change the name?
+* `RadioSet`: like `Stack` but for radio buttons: only has radio buttons and handles exclusivity automatically (this is also the only way to add radio buttons)
+* `Grid`: a grid of controls; they size themselves. Spacing is handled like `Stack`. (Analogues: Qt's `QGridLayout`)
+* `Form`: a set of label-control pairs arranged to resemble options on a dialog form. Sizing, positioning, and spacing are handled in an OS-dependent way. (Analogues: Qt's `QFormLayout`)
+
+## Windows
+There's only one (maybe two, if I choose to add floating toolboxes) window type. You can add one control to the content area of a window.
+
+In the case of dialogue boxes, you can call a function, say `RunDaialogue()` , that runs the dialogue modal, and adds standard OK/Cancel/Apply buttons for you.
+
+## An example
+``` go
+package main
+
+import (
+	"github.com/andlabs/ui"
+)
+
+func main() {
+	win := ui.NewWindow("Hello")
+	form := ui.NewForm()
+	name := ui.NewLineEntry()
+	form.Append("Enter your name:", name)
+	button := ui.NewButton("Click Me")
+	form.Append("", button)
+	win.SetControl(form)
+	
+	events, err := win.RunDialogue(ui.OkCancel)
+	if err != nil {
+		panic(err)
+	}
+	done := false
+	for !done {
+		select {
+		case event := <-events:
+			switch event {
+			case ui.Ok:
+				ui.MsgBox("Hi", "Hello, " + name.Text(), ui.Ok)
+			case ui.Cancel:
+				done = true
+			}
+		case <-button.Click:
+			ui.MsgBox("Hi", "You clicked me!", ui.Ok)
+		}
+	}
+	window.Close()
+}
+```