Add a shortcut to Global Theme in Edit Mode toolbar

This commit adds a button in Desktop Edit Mode Toolbox, to launch
the Global Theme KCM. This makes the setting more discoverable, and
easy to access when customizing the desktop.

toolboxes/desktoptoolbox/contents/ui/ToolBoxButton.qml

@@ -219,6 +219,11 @@ Item {
                 icon.name: "preferences-desktop-wallpaper"
                 onClicked: qAction.trigger()
             }
+            PlasmaComponents3.ToolButton {
+                text: i18nd("plasma_toolbox_org.kde.desktoptoolbox", "Choose Global Theme...")
+                icon.name: "preferences-desktop-theme-global"
+                onClicked: KQuickControlsAddons.KCMShell.openSystemSettings("kcm_lookandfeel")
+            }            
             PlasmaComponents3.ToolButton {
                 icon.name: "window-close"
                 Layout.preferredWidth: height