Drag & drop support

[itaib]

This PR adds support for drag & drop.
The mechanics mimic those of a file manager (KDE Dolphin, for example):

• A drag & drop operation is started when left clicking on an already selected row and dragging the mouse
• Starting a mouse drag from a row that is not selected:
  • If the mouse drag starts when clicking on the filename - a drag & drop operation will start.
  • Otherwise, it's a rubberband select operation
• Single clicking on a row will reset the selection to that row only on mouse release - so we'll be able to differentiate between single row selections and the beginning of a drag & drop event.