Support binding of left-click and right-click

left-click and right-click are respectively bound to "ignore" and
"toggle" (after implicitly moving the cursor) by default.

Close #1130

src/tui/tui.go

@@ -44,6 +44,8 @@ const (
 	Resize
 	Mouse
 	DoubleClick
+	LeftClick
+	RightClick
 
 	BTab
 	BSpace
@@ -185,6 +187,7 @@ type MouseEvent struct {
 	Y      int
 	X      int
 	S      int
+	Left   bool
 	Down   bool
 	Double bool
 	Mod    bool