This adds tab completion to textinput components. They can be configured
with a completion function. This function is called when the user
presses <tab>. The first completion is initially shown to the user
inserted into the text. Repeated presses of <tab> or <backtab> cycle
through the completions list. The completions list is invalidated when
any other non-tab-like key is pressed.
Also changed is some logic for current completion generation so that
all available commands are returned when <tab> is pressed with no
current text and similarly for arguments of commands.
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