Calculator is hard dependent on Qalculator

[fulalas]

When Qalculator is not present no error is given when we try to use the launcher calculator:

It would also be nice to have support to other ways of calculating things instead of depending on only one external application.

[mmstick]

I haven't yet seen any solutions that were as good as qalculate, so it is a hard dependency that packagers should be aware of to add as a runtime dependency.