Make IGFD::FileDialog's destructor virtual

[Jacajack]

Hi and thanks for the great library!

I noticed a compilation warning caused by the fact that IGFD::FileDialog has virtual member functions and a non-virtual destructor:

$ clang++ -c ImGuiFileDialog.cpp -I../imgui -Wall
ImGuiFileDialog.cpp:2972:3: warning: delete called on non-final 'IGFD::FileDialog' that has virtual functions but non-virtual destructor [-Wdelete-non-abstract-non-virtual-dtor]
                delete vContext;
                ^
1 warning generated.

While it's not critically important, I made the destructor virtual to avoid any trouble it might cause down the road. I hope this helps somehow :)

[aiekick]

ok thanks :)