Add NSPopUpButton class

[programmingkidx]

Adds the NSPopUpButton class. Use this example program to test out 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("NSPopUpButton")
        win.MakeKeyAndOrderFront(nil)

        /* NSPopUpButton code */
        rect := core.NSMakeRect(10, 40, 80, 25)
        popUp := cocoa.NSPopUpButton_Alloc()
        popUp.InitWithFrame(rect)
        popUp.AddItemWithTitle("One")
        popUp.AddItemWithTitle("Two")
        win.ContentView().AddSubview(popUp)
    })

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