[UX] Add home button (#3263)

Tap to go HOME. Hold to set current folder as HOME. When no HOME is set yet tap also asks to set current folder as HOME.

See https://github.com/koreader/koreader/issues/2957#issuecomment-308513062

Closes #3200.

frontend/ui/widget/touchmenu.lua

@@ -192,7 +192,8 @@ function TouchMenuBar:init()
             height = icon_height,
             scale_for_dpi = false,
             callback = nil,
-            horizontal_padding = icon_padding,
+            padding_left = icon_padding,
+            padding_right = icon_padding,
         }
 
         table.insert(self.icon_widgets, ib)