float the build buttons as a horizontal bar over the game world and show key bindings inside build buttons

src/ui/IconCaption.h

@@ -14,3 +14,9 @@
 // theme-correct; the result is devicePixelRatio-aware so text and icon stay crisp.
 QPixmap renderCaptionWithIcon(const QString& text, const QPixmap& icon,
                               const QFont& font, const QColor& textColor);
+
+// Item id of the building blocks resource, whose icon stands in for the "Blocks"
+// word wherever a cost or stock is captioned (REQ-UI-BLOCKS-ICON, REQ-UI-BUILD-COST,
+// REQ-UI-EXPAND-BUTTON). It lives next to the caption helper because every caller of
+// one is a caller of the other.
+const char* const kBlockItemId = "building_block";