Prevent the context menu from opening.

This frees up the right-click for us to use.

Closes #16
Added `rightClick` event for Tools.
Only pass left-clicks to the UI.
Zoom tool now makes use of `rightClick` events.

Also improves zoom’s dragging functionality.
Add posn conversions to lab
Use new conversion methods in Tool::buildEvent
Fix missing return value
Remove unneeded method
Merge branch 'context-menu' of https://github.com/Fishrock123/mondrian into right-click
Perform zoom transformation with cursor as origin

When I click a part of the image with the zoom tool,
the same part should be under my cursor after the
zoom is complete. Otherwise the zoom effect is disorienting.
Swap zoom tool's right-click and alt-click