* Implement fill tool
* Add fill tool shortcut
* Add getters and setters to styles
* Make fill tool act on the topmost layer clicked
* Refactor fill operation
* Refactor and unify selection tolerance
* Add mark_as_dirty function
* Fix getter names
* Implement nudging selected layers with arrow keys
* Normalize diagonal nudges and add constants
* Replace sqrt(2.0) with a constant
* Remove normalization of diagonal movement