*** New variable 'bookmark-bmenu-type-column-width'.

This variable allows a user to control the width of the type column
on the bookmark menu 'bookmark-bmenu-list'. The default value is 8
which is backwards compatible.

I marked this new variable as of 31.1 but could also be for 30.2.

Rationale: In one package I co-maintain, we have custom bookmark handlers
with appropriate properties set. The limit of 8 characters for display in
the bookmark menu can lead to cumbersome naming for handler abbreviations.

-Stephane