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

defaults.lua

@@ -41,6 +41,9 @@ DBACKGROUND_COLOR = 0
 -- outer page colour in scroll mode: 8 = gray, 0 = white, 15 = black
 DOUTER_PAGE_COLOR = 0
 
+-- generic icon size
+DGENERIC_ICON_SIZE = 40
+
 -- supported view mode includes: "scroll" and "page"
 DCREREADER_VIEW_MODE = "page"