Choose the readline defaults for the behavior of these two
functions/keybindings. Depending on the program, either of these can
delete the whole line.
Note that by default in [compose], <C-k> is bound to :prev-field<Enter>.
Leave it up to the user whether or not they want to rebind the key in
[compose].
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.
