Add click-header event for reporting clicks within header (#3768)

Sets $FZF_CLICK_HEADER_LINE and $FZF_CLICK_HEADER_COLUMN env vars with
coordinates of the last click inside and relative to the header and
fires click-header event.

Co-authored-by: Junegunn Choi <junegunn.c@gmail.com>

src/tui/tui.go

@@ -130,6 +130,7 @@ const (
 	Result
 	Jump
 	JumpCancel
+	ClickHeader
 )
 
 func (t EventType) AsEvent() Event {