emacs: Allow tuning of the tag/saved search layout.

Add `notmuch-column-control', which has three potential sets of
values:

- t: automatically calculate the number of columns per line based on
  the tags to be shown and the window width,
- an integer: a lower bound on the number of characters that will be
  used to display each column,
- a float: a fraction of the window width that is the lower bound on
  the number of characters that should be used for each column.

So:
- if you would like two columns of tags, set this to 0.5.
- if you would like a single column of tags, set this to 1.0.
- if you would like tags to be 30 characters wide, set this to
  30.
- if you don't want to worry about all of this nonsense, leave
  this set to `t'.

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