Support element-dependend cursor shape on hover

[papr]

It would be great if ui elements could tell the system to change the cursor shape on hovering it. E.g. GLFW_IBEAM_CURSOR for Text_Input or GLFW_HRESIZE_CURSOR for Trim Marks, etc

This would greatly improve the UX since users would notice areas of interaction that are currently hidden, e.g. draggable area of the Player seek handle.

We would need to extend handle_input to return a preferred cursor shape based on the input. The application managing the UI has to make the appropriate GLFW calls if the preferred cursor changes.

Further reading:

• http://www.glfw.org/docs/latest/input_guide.html#cursor_mode
• http://www.glfw.org/docs/latest/group__shapes.html

[mkassner]

Id rather work with the UI responding than the curser.

[papr]

Fixed with #88