LibGUI+FileManager: Clarify Widget margins name

Even though they are called content_margins,
they are actually only ever used to determine where
a Widget is supposed to be grabbable.
So all methods and members are renamed accordingly.

Userland/Libraries/LibGUI/MultiView.cpp

@@ -20,7 +20,7 @@ namespace GUI {
 MultiView::MultiView()
 {
     set_active_widget(nullptr);
-    set_content_margins(2);
+    set_grabbable_margins(2);
     m_icon_view = add<IconView>();
     m_table_view = add<TableView>();
     m_columns_view = add<ColumnsView>();