Hexagonal Grid Option

[doodlezucc]

User submitted feature request: Please implement the option to use hex grids and no grid at all.

Notes:

• Option "no grid" should be simple to code, but measuring has to be adjusted (emphasize exact area instead of cells/squares).
• Hexagonal grids are just square grids with shifted columns.

What needs to be done:

• [x] Research and implement measuring rules.
• [x] Shared calculation method to convert between squares and hexagons.
• [x] Add input field to select grid type (possibly as part of the scene settings).
• [x] Support Fog of War with every grid type.

[doodlezucc]

A dedicated Dart library for different grids and conversion between world and grid space was created at doodlezucc/dart_grid.

[doodlezucc]

Hex grids are now fully supported! Token movement and sizing, grid snapping, measuring (paths as well as all D&D 5e Areas of Effect) and a way to use Fog of War with hexagons have been implemented.

The changes include a refactor to how measurings are created and handled in code. Functionality of the grid library has been integrated and is visible in multiple parts of this repository as well as the dedicated Fog of War library (web_polymask).

All changes made while working on this issue can be seen in the GitHub diff.