HSBType/toString: limit precision of float value

This is needed to ensure that it can be stored in a database with a
row with a limited range of characters. E.g. errors like the following
are fixed with this patch:

0:43:17.985 ERROR o.o.p.m.i.MysqlPersistenceService[:317]- mySQL: Could not
store item 'gsAmbient' in database with statement 'INSERT INTO Item2
(TIME, VALUE) VALUES(NOW(),'217.085427135678478.0392156862745,100') ON
DUPLICATE KEYY UPDATE VALUE='217.0854271356784,78.0392156862745,100';': Data
truncation: Data too long for column 'Value' at row 1

and storing and reloading values of the Colorpicker Item works with the
MySQL backend.

Signed-off-by: Manuel Traut <manut@mecka.net>