[WIP]: integrate drop menu and filecopy drop menu

Summary:
add an optional menu parameter to processMimeData to mnually pass
an existing qmenu. folderview can use that to pass its own
filecopyjob drop qmenu to be populated with extra entries

Test Plan:
dropping a file on folderview will show a menu with
entries from both.
works both with folderview and desktop containments

Reviewers: Plasma, hein

Reviewed By: Plasma, hein

Subscribers: plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D4576

Change-Id: Iae911d4e692f2e591358959758ca6ddd131379c0