Please add 'const' to class member functions that are not modifying the object

[cr1st1p]

Example I bumped into, just now: DateTime::toShortDateString() Code does not change internal state of the object, so it should be declared as 'const', so that I'm allowed to use it on a 'const DateObject&'.

I think most of DateTime functions should be const.

Thank you.

[slaff]

@cr1st1p Can you make a PR with the suggested changes?

[frankdownunder]

-Wsuggest-attribute=const might help to find any functions that are candidates.to be marked const.

[slaff]

For the use of "const" we should make the changes across the whole Sming Framework classes and not just here and there. A "const" change should be merged with that PR #1384.