[ui] add "Controls" module to centralize common UI components

* add FloatingPane control + use it in Viewer3D

meshroom/ui/qml/Controls/FloatingPane.qml

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+import QtQuick 2.9
+import QtQuick.Controls 2.3
+import QtQuick.Layouts 1.3
+
+/**
+ * FloatingPane provides a Pane with a slightly transparent default background
+ * using palette.base as color. Useful to create floating toolbar/overlays.
+ */
+Pane {
+    id: root
+
+    property bool opaque: false
+
+    padding: 6
+    anchors.margins: 2
+    background: Rectangle { color: root.palette.base; opacity: opaque ? 1.0 : 0.7; radius: 1 }
+}

meshroom/ui/qml/Controls/qmldir

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+module Controls
+
+FloatingPane 1.0 FloatingPane.qml