Don't propagate unrecognized button clicks to the web engine

[elcuco]

There are a lot of unsupported mouse click events that the engine cannot handle. It such event (0) reaches the web engine - it will assert.

Don't even propagate them - this is the safe way. As of today!

I also added back/forward buttons to the translation.

NOTE that I removed one button related function. I am under assumption that we always need to send a bitmask. I may be wrong... in such case - I will revert part of this PR.

Should fix #27