Implement control: GuiFileDialog()

[raysan5]

It would be very useful to implement an advance control for file picking (open/save dialog). It could require a lot of work and dealing with OS system calls to read directory contents and info.

Proposed signatures could be:

const char *GuiFileDialog(const char *title, const char *currentPath);

Note that this control could be further customized for Open/Save file modes.

Immediate implementation is not required but just listing it here for future reference.

[raysan5]

A custom file dialog example has been added in commit d2d370e