Rename event handling methods from on* to handle*

[kraenhansen]

Historically, we've used a pattern naming methods that handle events with an on prefix (i.e. onClick, onCancel ...). We've decided that this is not the most meaningful naming and would instead like to rename these prefix handle instead. This is not an urgent issue, but it would be nice to get fixed before the code-base becomes two affected by this inconsistency in style.