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

* add FloatingPane control + use it in Viewer3D

meshroom/ui/qml/Controls/FloatingPane.qml

@@ -0,0 +1,17 @@
+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

@@ -0,0 +1,3 @@
+module Controls
+
+FloatingPane 1.0 FloatingPane.qml

meshroom/ui/qml/Viewer/Viewer3D.qml

@@ -5,8 +5,11 @@ import QtQuick.Scene3D 2.0
 import Qt3D.Core 2.1
 import Qt3D.Render 2.1
 import Qt3D.Input 2.1 as Qt3DInput // to avoid clash with Controls2 Action
+
 import MaterialIcons 2.2
 
+import Controls 1.0
+
 FocusScope {
     id: root
 
@@ -349,8 +352,8 @@ FocusScope {
     //
 
     // Rotation/Scale
-    Pane {
-        background: Rectangle { color: palette.base; opacity: 0.5; radius: 2 }
+    FloatingPane {
+        anchors { top: parent.top; left: parent.left }
 
         GridLayout {
             id: controlsLayout
@@ -391,9 +394,8 @@ FocusScope {
     }
 
     // Outliner
-    Pane {
-        anchors.right: parent.right
-        background: Rectangle { color: palette.base; opacity: 0.5; radius: 2 }
+    FloatingPane {
+        anchors { top: parent.top; right: parent.right }
 
         Column {
             Row {
@@ -430,9 +432,9 @@ FocusScope {
     }
 
     // Render Mode
-    Pane {
-        anchors.bottom: parent.bottom
-        background: Rectangle { color: palette.base; opacity: 0.5; radius: 2 }
+    FloatingPane {
+        anchors { bottom: parent.bottom; left: parent.left }
+
 
         Row {
             anchors.verticalCenter: parent.verticalCenter