Add NSComboBox class

[programmingkidx]

Add the NSComboBox class. This program can be used to test this pull request:

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("NSComboBox")
		win.MakeKeyAndOrderFront(nil)

		/* NSComboBox code */
		rect := core.NSMakeRect(10, 40, 80, 25)
		comboBox := cocoa.NSComboBox_Alloc()
		comboBox.InitWithFrame(rect)
		item1 := core.NSString_StringWithString("One")
		item2 := core.NSString_StringWithString("Two")
		comboBox.AddItemWithObjectValue(item1)
		comboBox.AddItemWithObjectValue(item2)
		win.ContentView().AddSubview(comboBox)
	})

	app.SetActivationPolicy(cocoa.NSApplicationActivationPolicyRegular)
	app.ActivateIgnoringOtherApps(true)
	app.Run()
}