Basically I want a UV-Grid image showing on the model without having to touch the material of the object. Whatever image I chose in the UV-Editor should be mapped to the object while I'm in edit mode. Is that possible? I'm running 2.8 now. Thanks
I believe it's not currently possible (as of December 2018) with a simple UI toggle. A UV checker overlay that doesn't affect the materials is one of the most loved features proposed by the community, but still not implemented.
mockup by Xuri Greer
I think that we will see this feature in the (very) near future, since it's known that the devs have discussed it already.
A workflow I normally use is:
Add a Shader editor to the "UV Editing" workspace, and switch the Viewport to LookDev
Make your actual material single user and create a different "Diagnostic material" with a textured Emission shader (add a UVMap input if it's not unique)
When you change the texture in the editor, update the material too
After having this set up, it's just a matter of one more click each time you switch.
(You may prefer using the "Node Wrangler addon"'s preview feature instead of a whole new material)