* first pass of adding grid snapping button
* update to request current grid size
* show/hide snapping option based on selection
* small tidy-up changes
* small updates following review feedback
* added some unit tests for snapping functionality and some small tidy-up/refactoring
* small refactor to ensure snap to grid ui only appears with snapping enabled
* add missing include to resolve build error
* fixes for build
* add & to make compiler happy
Tom Hulton-Harrop
alexpete