Revert supportTabs() removal to fix bookmenu issue

This reverts  0995c1dc95bb0df2a04aa84e1034044448ab542e; it is needed
to allow KBookMarks to know we supports tabs and thus show 'Open
folder in tabs' menu item.

FIXED-IN: 19.12.1
BUG: 415549

src/BookmarkHandler.cpp

@@ -144,6 +144,11 @@ QString BookmarkHandler::iconForView(ViewProperties *view) const
     return {};
 }
 
+bool BookmarkHandler::supportsTabs() const
+{
+    return true;
+}
+
 void BookmarkHandler::setViews(const QList<ViewProperties *> &views)
 {
     _views = views;

src/BookmarkHandler.h

@@ -68,6 +68,7 @@ public:
     QString currentTitle() const Q_DECL_OVERRIDE;
     QString currentIcon() const Q_DECL_OVERRIDE;
     bool enableOption(BookmarkOption option) const Q_DECL_OVERRIDE;
+    bool supportsTabs() const Q_DECL_OVERRIDE;
     void openFolderinTabs(const KBookmarkGroup &group) Q_DECL_OVERRIDE;
 
     /**