plugins: add PluginGlobalVar

- Allow syncing vars to all widget instances, like a singleton

Widgets/PluginGlobalVar.qml

@@ -0,0 +1,30 @@
+import QtQuick
+import qs.Services
+
+Item {
+    id: root
+
+    required property string varName
+    property var defaultValue: undefined
+
+    readonly property var value: {
+        const pid = parent?.pluginId ?? ""
+        if (!pid || !PluginService.globalVars[pid]) {
+            return defaultValue
+        }
+        return PluginService.globalVars[pid][varName] ?? defaultValue
+    }
+
+    function set(newValue) {
+        const pid = parent?.pluginId ?? ""
+        if (pid) {
+            PluginService.setGlobalVar(pid, varName, newValue)
+        } else {
+            console.warn("PluginGlobalVar: Cannot set", varName, "- no pluginId from parent")
+        }
+    }
+
+    visible: false
+    width: 0
+    height: 0
+}