Hello,

The attached patch adds a new user option to hide some minor mode
lighters on the mode line (by collapsing them into a menu), which is to
shorten the mode line and prioritize important information when many
minor modes are on.

There exist a few packages that solve the problem. To name a few:

- diminish[1] and delight[2]. Both offer a way to change the minor mode
  lighters by modifying 'minor-mode-alist'. To hide a minor mode
  lighter, one may change it to nil.

- minions[3], which partially inspired this patch. It replaces minor
  mode lighters by a menu to toggle all minor modes.

This patch is different from those solutions in two aspects:

- Unlike diminish or delight, one can still see the lighters by clicking
  the button, instead of hiding them permanently. Besides, this patch is
  compatible with those two packages.

- Unlike minions, this patch focuses on *lighters* for *enabled* minor
  modes. The menu contains only lighters, making it a more space
  efficient replacement for lighters on mode line, instead of a way to
  manage minor modes like minions.

So I hope this patch is still useful given the existing similar
solutions.

Please let me know what you think. Thanks!

Pengji

[1] https://github.com/myrjola/diminish.el
[2] https://elpa.gnu.org/packages/delight.html
[3] https://github.com/tarsius/minions