Add support for loading custom mouse cursors, provide missing ones on various platforms.

[rcoreilly]

requires platform-specific glue to create and set from image + mask + hotspot, and requires more robust asset provisioning strategy (same as icons -- currently not really addressed -- will add a new ticket for that too).

[rcoreilly]

glfw 3.3 has this -- but we need to actually initialize some standard ones. linux, windows pretty lame now I think.

[rcoreilly]

I looked for a while and couldn't find any good standard cursor sets -- lots of crazy custom ones -- hard to find key ones.

[rcoreilly]

done, see https://github.com/goki/cursors