Calculator runner

[Matteo Agostinelli]

Hello,

as suggested by Aaron in a previous discussion, I am having a look at the 
calculator runner with the purpose of extending the capabilities of the runner 
by using libqalculate when it is available, otherwise falling back to the 
currently used 'qscript' method. So here are my questions (sorry if some of 
them are stupid ones...):
* how should I detect the presence of qalculate on the system? Should I let 
CMake find the library and then use the preprocessor (#ifdef, ....) thus making 
it a compile-time optional dependency?
* it was mentioned that thread safety is important in this case. How can I find 
out if libqalculate is thread safe?
* I think it would be nice if the runner respects the same settings of the 
plasmoid (if it's being used). Is there a way to do this, apart from reading 
the plasma-desktop-appletsrc file?

BTW, while looking at the code I found a bug in the runner. The expression 
'e+' is interpreted as '^' instead of '*10^'. So if you type 2e+2 you 
currently get the wrong result (4 instead of 200). The same applies to 'e-'.