Function request GUI for filter equations

[erdi12]

Please add equations to the user interface of the filter. 
As a template, I suggest the user interface of the Synthetic Textures filter. 
The functions are:
1. Saving presets into categories
2. Loading presets
3. Managing presets
4. Saving variable presets
5. Definition of the variables as in the Synthetic Textures filter with 
user-specific names 0 to 1 variable, -1 to 1 variable, real number 
with rotating field, angle variables an so on.