Editor: implement find next/previous selected text.

Add actions to search next/previous occurence of the selected text/text
under the cursor, like in Visual Studio (ctrl+F3 and ctrl+shift+F3).

Task-number: QTCREATORBUG-464

Change-Id: I0bf44e386b91fce4b26c6e3864e6df484f2e3556
Reviewed-by: Andre Hartmann <aha_1980@gmx.de>
Reviewed-by: Leandro Melo <leandro.melo@nokia.com>
Reviewed-by: Yuchen Deng <loaden@gmail.com>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Eike Ziller <eike.ziller@nokia.com>