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/apps/reader/modules/readerdogear.lua

@@ -1,7 +1,7 @@
 local BD = require("ui/bidi")
 local Device = require("device")
 local Geom = require("ui/geometry")
-local ImageWidget = require("ui/widget/imagewidget")
+local IconWidget = require("ui/widget/iconwidget")
 local InputContainer = require("ui/widget/container/inputcontainer")
 local RightContainer = require("ui/widget/container/rightcontainer")
 local Screen = Device.screen
@@ -32,8 +32,8 @@ function ReaderDogear:setupDogear(new_dogear_size)
         end
         self[1] = RightContainer:new{
             dimen = Geom:new{w = Screen:getWidth(), h = self.dogear_size},
-            ImageWidget:new{
-                file = "resources/icons/dogear.png",
+            IconWidget:new{
+                icon = "dogear.opaque",
                 rotation_angle = BD.mirroredUILayout() and 90 or 0,
                 width = self.dogear_size,
                 height = self.dogear_size,