Exposing CanvasReference (#100)

for interop with style and attribute

Merge remote-tracking branch 'origin/master'

Blazor 3.0.0 support.

Some other fixes:
Fixes: #47
Fixes: #48
Fixes: #44
Fixes: #29

20190908.1

20190816.1

20190728.1

20190501.3

Upgraded to Blazor 0.7.0