Add an option to remove seconds hand in Clock activity

[llaske]

The Clock activity provide an option to let users set the clock by moving hands manually.
It's nice to learn children how to read time.

BTW it's complex for children, specifically for younger one, to set the seconds hand.
The idea of this enhancement is to add an option to hide/display the second hands.
The option could be a new selected/not selected button in the toolbar.
When the button is not selected the seconds hand is no longer visible in clock/set/play mode and in the digital display.

[llaske]

Fixed in 0aa669e