Add per-profile option 'BellMode' for choosing the type of bell

There are no GUI elements for controlling that option, so it is a hidden
option at the moment.

To configure this option manually, put following lines into some konsole
profile under ~/.kde4/share/apps/konsole/ :

    [Terminal Features]
    BellMode=N

Where N could be 0 for system beep, 1 for system notification and 2 for
visual bell. That mapping might change before next major release.

CCBUG: 155622
CCBUG: 177861

REVIEW: 104013