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/readermenu.lua

@@ -26,22 +26,22 @@ function ReaderMenu:init()
         },
         -- items in top menu
         navi = {
-            icon = "resources/icons/appbar.page.corner.bookmark.png",
+            icon = "appbar.navigation",
         },
         typeset = {
-            icon = "resources/icons/appbar.page.text.png",
+            icon = "appbar.typeset",
         },
         setting = {
-            icon = "resources/icons/appbar.settings.png",
+            icon = "appbar.settings",
         },
         tools = {
-            icon = "resources/icons/appbar.tools.png",
+            icon = "appbar.tools",
         },
         search = {
-            icon = "resources/icons/appbar.magnify.browse.png",
+            icon = "appbar.search",
         },
         filemanager = {
-            icon = "resources/icons/appbar.cabinet.files.png",
+            icon = "appbar.filebrowser",
             remember = false,
             callback = function()
                 self:onTapCloseMenu()
@@ -50,7 +50,7 @@ function ReaderMenu:init()
             end,
         },
         main = {
-            icon = "resources/icons/menu-icon.png",
+            icon = "appbar.menu",
         }
     }