fix: ban mouse click event in popup-window made by popup-message function.

[sakurawald]

To close issue: #1686

This commit does:

1. Add a new parameter clickable to window. (When clickable = nil, the mouse clicking event will be ignore for this window. However, thw mouse hover event and mouse scrolling event can still be received for that window)
2. Made the popup-window made by popup-message now un-clickable.

[jfaz1]

LGTM, maybe modify the comment to be a bit clearer:

;; NOTE: Block mouse clicks on popup windows to prevent them from gaining focus.
;; If a popup window gains focus, it becomes the only active window in the window-tree,
;; which prevents Lem from being able to close it.
;; Mouse scrolling remains enabled since popup windows are used for displaying text content.

[sakurawald]

LGTM, maybe modify the comment to be a bit clearer:

;; NOTE: Block mouse clicks on popup windows to prevent them from gaining focus.
;; If a popup window gains focus, it becomes the only active window in the window-tree,
;; which prevents Lem from being able to close it.
;; Mouse scrolling remains enabled since popup windows are used for displaying text content.

Modified.

[cxxxr]

Thank you