QmlDesigner: improve drag and drop inside navigator

Draw the last mouse over option rect and show a thicker line
while dragging.

Task-number: QTCREATORBUG-11723
Change-Id: I42685a2e2062f80dcb84fc11e693aeb0eee8a2b8
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
Put compiler prerequisites into README

Change-Id: I2567d6c06e7318c4aaccf3fd5dbbac5ecc0d456d
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>