Add NSLevelIndicator class

[programmingkidx]

Adds the NSLevelIndicator class. Use this program to test out the code:

package main

import "github.com/progrium/macdriver/cocoa"
import "github.com/progrium/macdriver/objc"
import "github.com/progrium/macdriver/core"

func main() {

	app := cocoa.NSApp_WithDidLaunch(func(n objc.Object) {
		win := cocoa.NSWindow_New()
		win.SetTitle("NSLevelIndicator")
		win.MakeKeyAndOrderFront(nil)

		/* NSLevelIndicator code */
		rect := core.NSMakeRect(10, 40, 80, 25)
		level := cocoa.NSLevelIndicator_Alloc()
		level.InitWithFrame(rect)
		/* Dependent on the "Add types to mapType" commit
		level.SetMaxValue(100)
		level.SetMinValue(1)  */
		level.SetIntValue(2)
		win.ContentView().AddSubview(level)
	})
	app.SetActivationPolicy(cocoa.NSApplicationActivationPolicyRegular)
	app.ActivateIgnoringOtherApps(true)
	app.Run()
}