When you enable filtering, the cursor enters the filter
field, and once you type a filter type, you have to get
the cursor out of the field by pressing tab or clicking
the mouse in order to use the inventory.
The keys used for filtering are mutually exclusive of
the characters used to select items in the inventory.
There is no ambiguity in determining what action the
user wants to take, so there is no need to let the text
field get the focus. Pressing the + key the first time
should add "+" to the filter list, and hitting it a
second time should remove it.