Calculator

ToDos

1. Implement a textbox to write math expression.
2. Create keypad (as floating window).
3. Math expression can be save as variable and function to create a table.

x1 = 100
x2 = 10 * 3
x3 = x1 + x2

f(x) = x + 2; -10 <= x <= 10
===================
|   x   |   f(x)  |
|=======|=========|
|  -10  |   -8    |
|  -9   |   -7    |
|  ...  |   ...   |
|   10  |   12    |
===================