Restore ComboBox

[zwcloud]

ComboBox is a common control and should be included in first alpha release. And it will be used by window rectangles inspector.

Popup window issues:

• [x] Popup window doesn't fill all space of the native window
• [x] Popup window doesn't respond to mouse click
• [x] Button rendered in popup window is incorrect
• [x] layout width is incorret: might be related to the node tree root width
• [x] #80 Text in popup window doesn't appear: text framebuffer size not updated
• [ ] implement proper focus behavior of a combo-box drop-down window, when it gets or loses focus

ComboBox issues:

• [x] Cannot re-open ComboBox pop up window by clicking the drop-down button: fixed at 32670de66e93f952cf5303fcb4b7e01e2a8e02bb
• [x] Rect got in popup window is incorrect, the position of client rectangle of Window should not be considered. Window.GetRect should return a rectangle in the window's local client space. This issue is related to layout of popup window nodes. the key point: combo box button and the popup dropdown list is owned by different native windows

This blocks #63