* Implement proper alpha-blending of SVG icons
Also, instead of doing that every time, cache a pre-composited version
that matches the screen's BB type.
This is faster, and also has the advantage of making icon highlights
behave.
Jot down a few notes about corner-cases or future improvements, e.g.,
dimming icons on non-white backgrounds, and nightmode with color 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
