Commandline: Add command to remove per-game grids and all game grids

[sonic2kk]

A small piece for #933.

This PR adds a command which can remove game grids, either for a specific AppID, or for all game grids by removing the grids folder (if it exists). This is a handy command for clearing out artwork for Non-Steam Games, or just in general if you download artwork you're unhappy with. Or, if your grids folder gets too big, you can clear it out.

I tested this and it works and is pretty safe. I did consider adding an extra guard around all the grids, such as a --yesimsure flag, or backing up beforehand, but I figured I'll go with trusting the user.

[sonic2kk]

Updated the all part of the command to re-create the grid folder, this fixes a minor warning we can end up with when fetching the show pic. It's harmless and there are some others like this we can end up with but figured one less is always good.

Shellcheck is green on this so we should be ready to merge.