Abacus activity should warn about data loss when changing styles

[greenfeld]

The Abacus activity should warn about data loss when changing styles.

Some of the styles are meant for large positive integers; others can go negative, while others support fractions.

If the new style supports representing the old value it is currently is kept; otherwise we start with an abacus set to show a value of zero.

Seen in OLPC 13.1.0 os2/Abacus-35.