Add bunch of ID's to qml components so it's easier to test them.

CURA-5618

plugins/Toolbox/resources/qml/Toolbox.qml

@@ -33,6 +33,7 @@ Window
         {
             id: header
         }
+
         Item
         {
             id: mainView
@@ -75,6 +76,7 @@ Window
                 visible: toolbox.viewCategory == "installed"
             }
         }
+
         ToolboxFooter
         {
             id: footer

plugins/Toolbox/resources/qml/ToolboxDownloadsGridTile.qml

@@ -9,6 +9,7 @@ import UM 1.1 as UM
 
 Item
 {
+    id: toolboxDownloadsGridTile
     property int packageCount: (toolbox.viewCategory == "material" && model.type === undefined) ? toolbox.getTotalNumberOfMaterialPackagesByAuthor(model.id) : 1
     property int installedPackages: (toolbox.viewCategory == "material" && model.type === undefined) ? toolbox.getNumberOfInstalledPackagesByAuthor(model.id) : (toolbox.isInstalled(model.id) ? 1 : 0)
     height: childrenRect.height

plugins/Toolbox/resources/qml/ToolboxHeader.qml

@@ -21,8 +21,10 @@ Item
             left: parent.left
             leftMargin: UM.Theme.getSize("default_margin").width
         }
+
         ToolboxTabButton
         {
+            id: pluginsTabButton
             text: catalog.i18nc("@title:tab", "Plugins")
             active: toolbox.viewCategory == "plugin" && enabled
             enabled: toolbox.viewPage != "loading" && toolbox.viewPage != "errored"
@@ -36,6 +38,7 @@ Item
 
         ToolboxTabButton
         {
+            id: materialsTabButton
             text: catalog.i18nc("@title:tab", "Materials")
             active: toolbox.viewCategory == "material" && enabled
             enabled: toolbox.viewPage != "loading" && toolbox.viewPage != "errored"
@@ -49,6 +52,7 @@ Item
     }
     ToolboxTabButton
     {
+        id: installedTabButton
         text: catalog.i18nc("@title:tab", "Installed")
         active: toolbox.viewCategory == "installed"
         anchors