Assigning mining tiles is unecessarily difficult

Issue description

Take any completely level area and set the mining tool to one tile lower than the currnet depth and flat.
Drag out an area.
Place the cursor to the edge of the set area and drag out another area (no issue).
Now move the cursor so it is not in contact wth the existing area. Oh dear, now it has decided that you want to mine one tile lower than what you set it at.
Now things get really out of hand, because if you didn't notice and do it again it will go another tile lower.
Even more stupid, if you now place the cursor between the two levels the game will decide that you really need a slope there...

Reproduction steps

Take any completely level area and set the mining tool to one tile lower than the current land height and flat.
Drag out an area.
Place the cursor to the edge of the set area and drag out another area (no issue).
Now move the cursor so it is not in contact wth the existing area. Oh dear, now it has decided that you want to mine one tile lower than what you set it at.

Now things get really out of hand, because if you didn't notice and do it again it will go another tile lower.
Even more stupid, if you now place the cursor between the two levels the game will decide that you really need a slope there...

Game version and build number

v0.7.9 b495

Operating system

Windows

I agree. I also wanted to suggest that if Q/E were pressed, it would automatically reset to 0 after the first designation.

The solution is simple, remove all the code that modifies the mining tool and let the user do the modifying.