Reduce margins of options in the open file drop-down

This was a desire from the developer. It shouldn't be consistent with the printer selector. The margins being all the same is more important.

Contributes to issue CURA-8008.

plugins/PrepareStage/PrepareMenu.qml

@@ -138,8 +138,8 @@ Item
                         model: prepareMenu.fileProviderModel
                         delegate: Button
                         {
-                            leftPadding: UM.Theme.getSize("thick_margin").width
-                            rightPadding: UM.Theme.getSize("thick_margin").width
+                            leftPadding: UM.Theme.getSize("default_margin").width
+                            rightPadding: UM.Theme.getSize("default_margin").width
                             width: contentItem.width + leftPadding + rightPadding
                             height: UM.Theme.getSize("action_button").height
                             hoverEnabled: true