Adds keyboard shortcuts for adding the currently-held item to the toolbar:

CONTROL + Q: Assign to first open slot.
CONTROL + [0-9]: Assign to corresponding slot. Without a held item, unassign that slot.

NOTE: Due to a bug in the Lua API, the CONTROL + Q shortcut will overwrite any blueprints. See also: https://forums.factorio.com/viewtopic.php?f=7&t=66002

Changelog:
0.4.0: Update for game version 1.0.
0.3.0: Update for game version 0.17.12 (see also 66142)
0.2.0: Initial working version.