[FEATURE] Support for function manipulation

[tommens]

Describe the new functionality you would like to see
This feature request involves storing and manipulating mathematical functions instead of just numerical values. The functions to be stored and manipulated would basically be arithmetic expression containing a single variable x (corresponding to mathematical functions f(x) with 1 argument x). For example, one could represent the following expressions: sin(x), (x^2)+(2*x)+1, (x-1)^2, exp(x^2) assuming that sin, exp, +, - and ^ are predefined functions known to the calculator.
Given such functions one could extend the calculator application to be able to plot/draw such functions on the screen. Optionally, this drawing functionality could be combined with zooming in and out, panning, and so on…
Optionally, in presence of functions, one could implement functionality for computing the derivative and the integral of a given function.