Version 21

The custom expression entering system was deleted cause it caused too many problems and didn't work properly anyways.
This is a more polished version 20 that looks nicer and is more stable.

The only known bug is plotting custom functions.
Plotting f[x] is fine, but plotting 2f[x] or f[x]+g[x] or f[g[x]] will not print the correct expression.
I don't feel like fixing it cause it is too much effort at this point.

Other than that, this version is rock solid.