Add mutexes to adding buttons and button image writes

[derickr]

When running AddButton in a go-routine, I received the following errors:

fatal error: concurrent map writes

goroutine 13 [running]:
github.com/magicmonkey/go-streamdeck.(*StreamDeck).AddButton(0xc000122a68, 0xe?, {0x8b68e0, 0xc0000aa050})
	/home/derick/dev/go/src/github.com/magicmonkey/go-streamdeck/streamdeck.go:59 +0xa7
github.com/derickr/streamdeck-goui/addons.(*Clock).Init.func1(0x6)
	/home/derick/dev/go/src/github.com/derickr/streamdeck-goui/addons/clock.go:70 +0x7b7
created by github.com/derickr/streamdeck-goui/addons.(*Clock).Init
	/home/derick/dev/go/src/github.com/derickr/streamdeck-goui/addons/clock.go:48 +0x30

It turned out that the method did not use a mutex for this.

Similarly, I frequently got corrupt images, and the addition of the mutex to the function that writes these images to the Stream Deck fixes that.