Better entity editor movement

[saint11]

It would be great to have some nice quality of life improvements on the entity selector. This would only be enabled if the entity is selected - otherwise, move camera, like we have it now:

• Arrow keys move entities by 1 pixel
• Shift+Arrow keys move entities by 1 tile.

Other features that would be nice:

• Hold shift while dragging constrain to an axis (horizontal, vertical).
• Alt+Click+Drag duplicate entity and drag.

[saint11]

Nice, thanks @grouting!
The only remaining feature we need now is Alt+Click+Drag duplicate entity and drag.

[grouting]

I'll start work on this as soon as possible.

[saint11]

There are some interactions with the collider editor that are conflicting with the Shift+Drag (axis drag) I'll fix it here

[grouting]

I've made a PR (#76) that fixes an issue with the axis drag when multiple entities are selected.