[notifications] Refactor the screen handling code

This patch does this:
* moves the screen handling code from the import into
the applet baseclass, which can access the containment
available screen rect and watch for screen changes

* fixes the applet config dialog's custom screen position
setting which has a bug of always being enabled

* consolidates duplicated code in the helper import
into functions

* ensures that popups have correct positions when on
screen that does not start with y=0 (and x=0 in some
cases too)

REVIEW: 126408
CCBUG: 353966
CCBUG: 356461