[Icons] Migrate UI glyphs to palette-token SVGs

Runtime inversion is not a theming system: it rotates hue, breaks duotone art
and mishandles alpha, and a QPixmap snapshot cannot follow a palette change.
Redraw the top-level UI glyph set on a 24x24 grid with `currentColor` as the
paint token, drop Inkscape/Sodipodi cruft, and delete the icons whose metaphor
was duplicated (cogwheel/gear -> settings, scale_balanced -> scales,
update.png -> reload).

Load the glyphs through `loadThemedIcon` so the engine resolves the token from
the palette on every request, and let `themePixmap` use the same resolution for
one-shot pixmaps. `loadColorAdjustedPixmap` is gone; every caller now keeps a
QIcon, which `SettingsButtonWidget::setButtonIcon` accepts instead of a QPixmap.

Content art (mana, counters, countries) keeps its authored colors.

Add a render test covering aspect ratio, palette flips, icon modes and
high-DPI pixmaps.

cockatrice/resources/icons/dropdown_collapsed.svg

@@ -1,10 +1,3 @@
-<?xml version="1.0" encoding="utf-8"?>
-<!-- Generator: Adobe Illustrator 12.0.0, SVG Export Plug-In . SVG Version: 6.00 Build 51448)  -->
-<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd" [
-        <!ENTITY ns_svg "http://www.w3.org/2000/svg">
-        <!ENTITY ns_xlink "http://www.w3.org/1999/xlink">
-        ]>
-<svg version="1.1" id="Layer_1" xmlns="&ns_svg;" xmlns:xlink="&ns_xlink;" width="460.5" height="531.74"
-     viewBox="0 0 460.5 531.74" overflow="visible" enable-background="new 0 0 460.5 531.74" xml:space="preserve">
-<polygon fill="#918d8d" points="0.5,0.866 459.5,265.87 0.5,530.874 "/>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+<path d="M9 6l6 6-6 6"/>
 </svg>