Add support of drag-and-drop to open files

Summary:
Add support of drag-and-drop to open files.
When first open okteta, drag files to the blank view area to open files

Test Plan: drag and drop to open files

Reviewers: kossebau

Reviewed By: kossebau

Subscribers: bcooksley

Differential Revision: https://phabricator.kde.org/D10033