Reapply build button grid icons

Revert of "Undo build button grid icons" (614dda8): restore the per-building
SVG icons, the QtSvg-based icon loading in BuildButtonGrid, the icon dir
plumbing in MainWindow, and the Qt5::Svg dependency.

This returns the unlock-groups branch to having the icons. The preceding
undo commit exists only so the Demolish -> Deconstruct rename can be
cherry-picked on its own (rename+icons commit together with the undo).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y7N59FsLA5e2kuVdqe4Uhc

bin/app/data/icons/buildings/tunnel_entry.svg

@@ -0,0 +1,6 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100" width="100" height="100">
+  <rect width="100" height="100" rx="22" fill="#4f7562"/>
+  <g transform="translate(13,13) scale(2.3125)" fill="none" stroke="#ffffff" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
+    <path d="M4 9h24"/><path d="M8 9C8 24 24 24 24 9"/><path d="M14.5 18.5 17.5 20 14.5 21.5"/>
+  </g>
+</svg>