Bug tracker plotting categories.

[Pantxo Diribarne]

Hello,

With the recent integration of Qt graphics toolkit, there will emerge GL 
toolkit specific bugs. Theres is already an example here [1], and it 
would help having dedicated categories in the bug tracker.

If you agree with this, I think the best is simply adding two categories 
"Plotting with Qt" and "Plotting with FLTK".  I initially thought 
"Plotting with OpenGL" could be split into two but we wouldn't know 
where to put bugs that are common to qt and fltk.

Pantxo

[1]  https://savannah.gnu.org/bugs/?41573