For the work on T113243, the Popups extension should be structured in a
more flexible way. This is achieved with this change.
Following has changed:
* The main/core logic of Popups now lives in ext.popups.core to be shareable
between implementations
* The desktop specific logic lives in ext.popups.desktop now, the frontend init
code in ext.popups.desktopTarget and the desktop renderer in
ext.popups.renderer.desktopRenderer
This change doesn't change the functionality of Popups.
Change-Id: I72121e0a1e4b2952f85dc1bc8cf59d06b8d22f47
