Add a way to precisely move, scale, and rotate stencils in Texture paint mode.

In the viewport it's currently possible to modify a stencil's masks:

• position with ALT + Right click
• scale with alt + shift + right click
• rotation with alt + ctrl + right click

But unlike most tools in Blender there is no way to get a precise control over this transformations, you are just stuck with whatever Blender think is a good speed for scaling, rotating, etc... This can be dampened with your zoom level but if you have a big stencil you can't get too close to your object.

I'd suggest to add the shift key as a precise control once you release the key combination but are still holding on right click. This would help a lot with big stencils placement.

PS: I believe this is valid for sculpt mode too, even though it seems currently bugged in 2.81 as I'm not able to move the stencil at all with this kind of keymaps (which are the same than for texture painting, I've checked)