search for a default wallpaper in lookandfeel

Summary:
now look and feel packages can specify a default image wallpaper.
when a new containment is created at first plasma start or at activity creation
search for a default wallpaper inthe order:
look and feel package
if fails, from the plasma theme

we are sure to find a default after it as if neither of those specify one,
the wallpaper specified by the breeze plasma theme (Next) will be used

D21225 takes care of setting the wallpaper when a lnf theme is selected by the kcm

Test Plan:
tested to start plasma with an empty config file with different themes
that do or don't include wallpapers

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: kossebau, davidedmundson, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D21226