"shift click" should stack items if stacking would be available

[grepsedawk]

• When I have a stack of 25 blocks in my inventory and I shift click 4, it should add 4 to the stack of 25
• When I have a stack of 25 and I shift+click a stack of 64 it should "fill" the 25 to 64 and ALSO move a stack of 39 over to the inventory
• When I have 2 stacks of 60 and I shift+click a stack of 64 it should fill both 60's up to 64 and transfer a new stack of 4
• (existing) when I have a stack of 64 in my inventory and I shift+click a stack of 4, it should make a new stack of 4 in my inventory