feat: Rewrote inventory handling basing on ESCItems (#417)

Co-authored-by: Dennis Ploeger <develop@dieploegers.de>

addons/escoria-core/design/esc_inventory_item.svg

@@ -1,27 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<svg width="16px" height="16px" viewBox="0 0 16 16" version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
-    <title>esc_inventory_item</title>
-    <defs>
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-            <stop stop-color="#D4FF2A" offset="0%"></stop>
-            <stop stop-color="#81D135" offset="100%"></stop>
-        </linearGradient>
-        <linearGradient x1="0%" y1="0%" x2="100%" y2="100%" id="linearGradient-2">
-            <stop stop-color="#D4FF2A" offset="0%"></stop>
-            <stop stop-color="#81D135" offset="100%"></stop>
-        </linearGradient>
-    </defs>
-    <g id="esc_inventory_item" stroke="none" stroke-width="1" fill="none" fill-rule="evenodd">
-        <g id="Group-Copy" transform="translate(2.000000, 4.000000)">
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-        <path d="M13,2 L13,12 L3,12 L3,2 L13,2 Z" id="Rectangle-Copy" fill="url(#linearGradient-2)" fill-rule="nonzero"></path>
-        <path d="M13,2 L13,12 L3,12 L3,2 L13,2 Z M12,3 L4,3 L4,11 L12,11 L12,3 Z" id="Rectangle" fill="#1B2F0D" fill-rule="nonzero"></path>
-        <g id="Group" transform="translate(1.000000, 6.000000)">
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-</svg>