[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/buttonprogresswidget.lua

@@ -22,6 +22,7 @@ local ButtonProgressWidget = InputContainer:new{
     default_position = nil,
     thin_grey_style = false, -- default to black
     fine_tune = false, -- no -/+ buttons on the extremities by default
+    more_options = false, -- no "⋮" button
 }
 
 function ButtonProgressWidget:init()
@@ -50,6 +51,9 @@ function ButtonProgressWidget:update()
     if self.fine_tune then
        buttons_count = buttons_count + 2
     end
+    if self.more_options then
+        buttons_count = buttons_count + 1
+    end
     local button_width = math.floor(self.width / buttons_count) - 2*button_padding - 2*button_margin - 2*button_bordersize
 
     -- Minus button on the left
@@ -169,6 +173,35 @@ function ButtonProgressWidget:update()
         end
         table.insert(self.buttonprogress_content, button)
     end
+    -- More option button on the right
+    if self.more_options then
+        local margin = button_margin * 6
+        local extra_border_size = 0
+        local button = Button:new{
+            text = "⋮",
+            radius = 0,
+            margin = margin,
+            padding = button_padding,
+            bordersize = button_bordersize + extra_border_size,
+            enabled = true,
+            width = button_width - 2*extra_border_size,
+            preselect = false,
+            text_font_face = self.font_face,
+            text_font_size = self.font_size,
+            callback = function()
+                self.callback("⋮")
+                self:update()
+            end,
+            no_focus = true,
+            hold_callback = function()
+                self.hold_callback("⋮")
+            end,
+        }
+        if self.thin_grey_style then
+            button.frame.color = Blitbuffer.COLOR_DARK_GRAY
+        end
+        table.insert(self.buttonprogress_content, button)
+    end
 
     UIManager:setDirty(self.show_parrent, function()
         return "ui", self.dimen