After reading this, I created a mod for the game which adds a hotbar (https://qe.mod.io/hotbar).
You can install this mod from the main menu of the game. Just click on the mods button at the main menu and click subscribe under the hotbar mod.
The hotbar has 8 slots.
You can drag items from the inventory screen onto the hotbar or hover over them with the mouse cursor and press the corresponding number key (1-8) on your keyboard to assign them to a slot.
Hotbar slots can be selected with the number keys on your keyboard (1-8) or by scrolling with the mouse wheel.
You can see the hotbar in use in the gif below:
