Cleanup classes naming convention

[JFlynnXYZ]

button.h, io_xml.h, project.h and shadermanager.h, json.h are all lower case. Need to have them match the NGL coding standard.

• [x] button.h, button.cpp @jlyddon
• [x] io_xml.h, io_xml.cpp @EllieAnsell
• [x] project.h, project.cpp @mainConfetti
• [x] shadermanager.h, shadermanager.cpp @mainConfetti
• [x] json.h, json.cpp @EllieAnsell

[JFlynnXYZ]

Just @jlyddon and @mainConfetti to finish this.

[jlyddon]

Nearly done, need to do a touch more of this as I have added some functions, should be done by the end of today

[mainConfetti]

all my stuff is clean and documented :D