Added the ability to change grid type in Preferences, and made an isometric cell size value which affects just the size of the cells of the isometric grid. Also updated translations.
I mostly did it so I could move tile mode logic into a new node, so I could have control over the drawing order. TileMode.gd also always draws 8 blank rectangles, which are being drawn on top of the grid, in order to hide it if it gets out of boundaries. We are now very close to implementing an isometric grid, #305.
This shouldn't have much of a performance impact, but some more testing is most likely needed.
