Fix copy/paste in ctkConsole

In addition to support paste from Ctrl+V, it now supports any paste
(e.g. with middle mouse button).
Also, the behavior is changed to now support Ctrl+V even when
the cursor is in the history area. Pasted text is appended at the end of
the document.

Closes #294
Merge pull request #366 from finetjul/294-fix-ctkconsole-copy-paste

Fix copy/paste in ctkConsole