Show empty wear bar when tool needs charging

[OgelGames]

From the pandorabox #ideas channel:

FeXoR
Always make the charge state of an item visible e.g. by showing a black bar instead of transparance as background
(To comunicate more clearly to the player that this item is empty and needs charging)
SX
Maybe also some simple nice generic sounds if/when trying to use power tool without enough charge.

In addition to showing the wear bar after being used, I think it would also be a good idea to show it on newly crafted tools. This could be done with minetest.register_on_craft, or by adding the metadata to the item in each craft recipe.