Summary: Make it possible to drag multiple clips to the timeline

Test Plan:

• Add clips to the project bin
• Select two or more clips
• Drag them to the timeline
• Check that the "Insert Clips" entry got added to the undo history
• Undo the change
• Redo the change

Reviewers: alcinos, mardelle

Reviewed By: alcinos, mardelle

Differential Revision: https://phabricator.kde.org/D8928