Update UI layout code to use new SVG icons

- Add IconWidget, use it for icons instead of ImageWidget.
  Specify icons by name only, look for them (with either
  .svg or .png suffixes) in multiple directories (including
  koreader/settings/icons/ to allow customizing them).
  Don't crash when icon name not found, shown a black
  background warning icon instead.
- Don't trust the icons' native sizes: replace
  scale_for_dpi=true with width/height=DGENERIC_ICON_SIZE,
  so all icons get the same (tunable) size - except in
  a few specific use cases.
- Top and bottom menu bars: normalize, and have icons
  properly centered in them, extend vertical line
  separators up to the edges.
- TOC: adjust expand/collapse icons size to items size

frontend/ui/widget/bookstatuswidget.lua

@@ -91,7 +91,7 @@ function BookStatusWidget:init()
     end
 
     self.star = Button:new{
-        icon = "resources/icons/stats.star.empty.png",
+        icon = "star.empty",
         bordersize = 0,
         radius = 0,
         margin = 0,
@@ -256,7 +256,7 @@ function BookStatusWidget:setStar(num)
 
         for i = 1, num do
             table.insert(stars_group, self.star:new{
-                icon = "resources/icons/stats.star.full.png",
+                icon = "star.full",
                 callback = function() self:setStar(i) end
             })
         end