Replaced sprintf calls with dedicated code
Using sprintf generates a large amount of code. By replacing the
sprintf calls with dedicated code, program size is reduced when
widgets are used.

Also:
- Added a getValLen method for widgets which returns the print
length for the current value, similar to the existing getMaxValLen.
- The value for widgets is now initialised to the minimum value,
instead of 0, because 0 could be outside the specified range.
- Added getValLen and getMaxValLen keywords.
- The MicroViewWidgetDemo example sketch was modified to use
the new getValLen method, plus a few other small changes.
Merge pull request #20 from MLXXXp/replace_sprintf

Replaced sprintf calls with dedicated code