Add a "new-message-bell" option to the UI section of aerc.conf. A new
hook into the message store allows the msglist widget to detect new
messages being added to the displayed list. When new messages are
delivered, and the new-message-bell option is enabled (as it is by
default), the terminal will beep.

Tabstrip didn't take into account the width of the context. Now, it just
shows as many tabs as can fit and truncates the last one if necessary.

In future it probably would be best to ensure that the selected tab is
rendered on the screen.

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.

Executing :close on a terminal would panic due to it already having been
removed.

This is also related to the fact that removing a tab doesn't check for
whether it actually found a tab to remove or not.

The grid was not checking there was enough space for the cells so would
just attempt to create subcontexts that don't actually fit.

This attempts to use the remaining space and then if there is no space
then it just skips drawing this cell.

This command allows the user to change tab by giving the tab name. This
can be tab completed too. The previous tab is stored in the tabs module
so that when a new tab is created it is still possible to go to the
previous one.

Normal invocation is :ct folder
Previous tab is :ct -

If the column weights do not collectively divide the extent of the grid
layout then some width was not used and so would not be redrawn,
resulting in previous drawings showing through.

This fixes this by checking if there is any remainingExact width and if
there is it is assigned to the weighted columns by their proportion from
left to right.