Let user specify the alarm message

When the alarm fires we show a dialog box with the string "Alarm". It
would be useful if a custom message could be picked. Add a text view
that allows the user to change the message into something else.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Add constraint below clock to super view

There is no constraint set between the clock and the super view. Due to
that if the window height is reduced to the minimum then parts of the
clock might no longer be visible.

Add a constraint below the clock that makes sure the default minimum
space between the clock and the super view is maintained.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>