Add class defaultGetTexture variable
Throw exception if textures not specified
Add ability to use global.InventoryWindow_defaultGetTexture for textures

Trying to make inventory-window easier to use, but when setting textures
there is a layering problem. Initially, you always had to pass the getTexture
option, meaning any module wanting to use inventory-window had to know about
textures, so it had to be passed this information then pass it on.. included
at least: voxel-inventory-hotbar, voxel-inventory-dialog, voxel-workbench.

Introduced the concept of "default getTexture", set with the class variable
InventoryWindow.defaultGetTexture, used of opts.getTexture unspecified. However,
in a browserified environment, for some reason, assigning InventoryWindow.defaultGetTexture
in the top-level application module (voxpopuli) doesn't affect InventoryWindow.defaultGetTexture
included in submodules (those previously mentioned). "Class variables", set on
the class object, are not global enough to reach all the interested code.

To solve this, lastly check global.InventoryWindow.defaultGetTexture. It is truly
global so can be set from anywhere and will make its way down to any dependent module.