Mac users, especially those of us with a non-US keyboard, need the Option keys for entering various characters, and thus set 'ns-alternate-modifier' to nil, or use 'ns-right-alternate-modifier' to employ only one of them for this purpose. However, this prevents use of that key as modifier for function keys (arrows, backspace etc) or mouse actions.

This patch adds the variable 'ns-alternate-modifier-char-entry' which, when non-nil, inhibits the Option key translation to Emacs modifiers for character keys but allows it for any other event. For example, setting

(setq ns-alternate-modifier 'super)
(setq ns-alternate-modifier-char-entry t)

makes Option act as Super with function keys and mouse clicks, but retains the standard macOS semantics for character entry (for example, Option + p = π).