Use Cura.Menu instead of UM.Menu

Same as the previous commit. Use the specialised variant provided by Cura instead
of the generic one of UM

CURA-8683

resources/qml/Menus/ExtensionMenu.qml

@@ -27,7 +27,7 @@ Menu
     {
         id: extensionsMenuSeparator
 
-        MenuSeparator {}
+        Cura.MenuSeparator {}
     }
 
     Instantiator
@@ -35,7 +35,7 @@ Menu
         id: extensions
         model: extensionModel
 
-        UM.Menu
+        Cura.Menu
         {
             id: sub_menu
             title: model.name

resources/qml/Menus/MaterialMenu.qml

@@ -7,7 +7,7 @@ import QtQuick.Controls 2.4
 import UM 1.5 as UM
 import Cura 1.0 as Cura
 
-UM.Menu
+Cura.Menu
 {
     id: materialMenu
     title: catalog.i18nc("@label:category menu label", "Material")

resources/qml/Menus/NozzleMenu.qml

@@ -7,7 +7,7 @@ import QtQuick.Controls 2.4
 import UM 1.5 as UM
 import Cura 1.0 as Cura
 
-UM.Menu
+Cura.Menu
 {
     id: nozzleMenu
     title: "Nozzle"

resources/qml/Menus/OpenFilesMenu.qml

@@ -9,7 +9,7 @@ import Cura 1.0 as Cura
 
 import "../Dialogs"
 
-UM.Menu
+Cura.Menu
 {
     id: openFilesMenu
     title: catalog.i18nc("@title:menu menubar:file", "Open File(s)...")

resources/qml/Menus/SaveProjectMenu.qml

@@ -9,7 +9,7 @@ import Cura 1.1 as Cura
 
 import "../Dialogs"
 
-UM.Menu
+Cura.Menu
 {
     id: saveProjectMenu
     title: catalog.i18nc("@title:menu menubar:file", "Save Project...")

resources/qml/Widgets/Menu.qml

@@ -2,20 +2,18 @@
 // Cura is released under the terms of the LGPLv3 or higher.
 
 import QtQuick 2.7
-import QtQuick.Controls 2.3
 
-import UM 1.1 as UM
+import UM 1.5 as UM
 
 //
 // Menu with Cura styling.
 //
-Menu
+UM.Menu
 {
     id: menu
     padding: 0
 
     implicitWidth: UM.Theme.getSize("setting_control").width
-    width: Math.max.apply(Math, Object.values(contentChildren).map(function(c) { return c.width }))
 
     background: Rectangle {
         color: UM.Theme.getColor("setting_control")