[WIP]: unify file drop menu and containment

Summary:
very early version of unification of the desktop and folderview drop
menus: use the instance of the qmenu created by the filecopy job, and
send it to the containment to be populated by the extra entries to
create plasmoids/apply wallpaper

Test Plan:
tested only for folderview, work in progress to make it work
with normal desktop containment

Reviewers: hein, #plasma

Reviewed By: hein, #plasma

Subscribers: hein, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4574