Update Popup Implementation in Neptune 3 to support Application Popup

As a developer, I would like to have a way that applications should be able to trigger system modal popups.

These are multi-step (and hence multiple commits) tasks need to be done:

1. Rename NeptunePopup to PopupItem
2. Create a new component called PopupWindow, which inherits from ApplicationManagerWindow (just like PrimaryWindow and SecondaryWindow). PopupWindow will have a window property called "windowType" whose value is "popup". system ui will take care of showing this window at the appropriate layer and of summoning the blurred background behind it.
3. There shall be an example in the sheets app making use of PopupWindow, alongside the existing PopupItem one. Both PopupItem and PopupWindow should be properly documented, particularly regarding the difference between the two.