#164 Default function label reinserted if the function is edited

[Fellype]

The default function label is reinserted in the legend box for each time the function is edited.
Example:
1. Create any function plot, say x. The default function's label will be F1 and the legend box will show "F1: x"
1.1 *
2. Edit something in the function (the x range for e.g.) -> Now the legend box shows "F1F1: x"

The same occurs for parametric and polar plots.

* Optionally, you can change the label F1, calling it "my_function". This don't solve the problem.