Fix: Mex building deselection in default buildmenu

[thehobojoe]

Work done

Buildmenu doesn't do any shift handling, but it still behaves like this
If using the default build menu:

• with a building selected, you can press and release shift and nothing happens
• With a building selected, you can place a building without shift, it will be queued and the building will be deselected
• With a building selected, you can place it with shift, it will be queued and the building will remain selected
• AFTER placing a building with shift, releasing shift will deselect the building.

I cannot find any code that matches the described behavior here, so I suspect it's set in the engine somewhere. I manually handle shift and building deselection in extractor_snap to get around this.