This adds the Mouseable interface. When this is implemented for a
component that item can accept and process mouseevents.
At the top level when a mouse event is received it is passed to the
grid's handler and then it trickles down until it reaches a component
that can actually handle it, such as the tablist, dirlist or msglist.
A mouse event is passed so that components can handle other things such
as scrolling with the mousewheel. The components themselves then perform
the necessary actions.
Clicking emails in the messagelist opens them in a new tab.
Textinputs can be clicked to position the cursor inside them.
Mouseevents are not forwarded to the terminal at the moment.
Elements which do not handle mouse events are not required to implement
the Mouseable interface.
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.
