BUG 14388: Fixed bug when text is pasted in with mouse middle-click

Middle-mouse-clicking to paste text into the Python console was broken. The problem was that text was being inserted into the document before the return key was pressed, so the position of the cursor was effectively at the end of the pasted text. Hence, the console thought there was no command to execute.

Fixed this by overiding mouseReleaseEvent(QMouseEvent* e), resetting the cursor position to its position prior to the middle-click, and updating the CommandHistory member variable with the pasted text.