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/renderimage.lua

@@ -248,7 +248,7 @@ function RenderImage:renderSVGImageFileWithNanoSVG(filename, width, height, zoom
     local bb = Blitbuffer.new(width, height, Blitbuffer.TYPE_BBRGB32)
     svg_image:drawTo(bb, zoom, offset_x, offset_y)
     svg_image:free()
-    return bb
+    return bb, true -- is_straight_alpha=true
 end
 
 function RenderImage:renderSVGImageFileWithMupdf(filename, width, height, zoom)
@@ -293,7 +293,7 @@ function RenderImage:renderSVGImageFileWithMupdf(filename, width, height, zoom)
     end
     page:close()
     document:close()
-    return bb
+    return bb -- pre-multiplied alpha: no is_straight_alpha=true
 end
 
 return RenderImage