MessageList: load icons in the size that is actually used by the Theme
Improves looks and performance. Previously we had a single cache of icon pixmaps in Manager. Manager always loaded 64x64 icons, even if the theme was using 16x16 (which is the default). That resulted in ugly icons, both due to down-scaling while rendering, and because we were using icons designed to be shown as 64x64, not 16x16 or similar. This change moves the pixmap cache to Theme, so that each Theme has its own cache using the Theme's icon sizes. The cache is populated lazily, so unused themes don't waste memory with cache. It also slightly improves performance, because the pixmaps don't have to be scaled down when rendering.
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