Add Hexagonal Isometric projection

Currently Tiled supports Hexagonal maps with offset coordinates. There are other possible coordinate systems, and one of them is Axial coordinates. This can actually be used already in Tiled using the Isometric projection, as illustrated by Galdred on the forum:

This issue is about adding a new "Hexagonal (Isometric)" or "Hexagonal (Axial)" (not sure about the best name) projection that derives from the IsometricRenderer and performs things like grid rendering and mouse picking correctly for hexagonal tiles.

Hopefully we can re-use the existing hexagonal map related properties, though their names would be a bit confusing (whether the hexagons are flat topped or pointy topped depends on the "stagger axis" property, and there is no staggering axis when using axial coordinates). The property could be renamed to "Hexagon Orientation" with values being either Horizontal / Vertical or Flat Topped / Pointy Topped.

Start by reading the existing IsometricRenderer and the current hexagonal map behavior to understand how grid rendering and mouse picking are handled. Define the projection name and hexagon-orientation semantics before implementation; done means hexagonal axial maps render their grid and support correct mouse picking.