[applets/calculator] Use Qt's in-built number formatting
Gets rid of a load of code.
Correctly handles floating point issues.
Results are correct.
Potentially this inadvertedly introduces a behaviour change
of including group separators depending on locale. This could be
considered a good thing, but if it's problematic we can always add a
.replace() at the end of this line.
62 * 1.3 = 80.6
100000000 * 5000000 = 5e+17
(with whatever the right number of zeroes is)
42 * 1.33333 = 6.6666665
Differential Revision: https://phabricator.kde.org/D26676