[UX] Add fine tune in line spacing (#5375)

See https://github.com/koreader/koreader/issues/5312#issuecomment-528506421

Progress widget has a new extra button ⋮ that triggers SpinWidget.

frontend/ui/widget/spinwidget.lua

@@ -33,6 +33,9 @@ local SpinWidget = InputContainer:new{
     value_hold_step = 4,
     ok_text = _("OK"),
     cancel_text = _("Cancel"),
+    -- extra button on bottom
+    extra_text = nil,
+    extra_callback = nil,
     -- set this to see extra default button
     default_value = nil,
     default_text = _("Use default"),
@@ -141,6 +144,20 @@ function SpinWidget:update()
             },
         })
     end
+        if self.extra_text then
+            table.insert(buttons,{
+                {
+                    text = self.extra_text,
+                    callback = function()
+                        if self.extra_callback then
+                            self.value = value_widget:getValue()
+                            self.extra_callback(self)
+                        end
+                        self:onClose()
+                    end,
+                },
+            })
+    end
 
     local ok_cancel_buttons = ButtonTable:new{
         width = self.width - 2*Size.padding.default,