[Task Manager] Allow dropping files onto tasks

This allows to drop files onto entries in the task manager.
Both single and multiple files are supported.

When all dropped items are desktop files for applications, such as dragging apps
from Kickoff, a launcher is created for each item.
Otherwise the application is launched with those URLs as arguments and behavior
depends on the application. It might open the file(s) in new tab(s) of an existing
instance, open a new window with tabs, open new windows for each files, etc.

BUG: 365626
FIXED-IN: 5.8.0

Differential Revision: https://phabricator.kde.org/D2392