include: vlc_list.hpp: add insertion/removal functions

Add some methods to replace their C counterpart when using a vlc_list,
so that elements can be added and removed. Those methods are only
supported on a vlc::list<T> wrapper, but they are not supported neither
on the vlc::const_list<T> wrapper (which is expected) nor on the
vlc::list::reverse_list wrapper from ::as_reverse() function, because it
would complexify the design.

The current workaround that was chosen is that vlc::list<T> is able to
use iterators from the reversed list, and can be swapped with the
reverse list object where needed.

The limitation will be lifted with C++20 and concepts.

Co-authored-by: Alexandre Janniaux <ajanni@videolabs.io>

include/vlc_list.hpp

@@ -339,6 +339,11 @@ public:
     {
         return a._list != b._list;
     }
+
+    bool empty() const
+    {
+        return vlc_list_is_empty(_list);
+    }
 };
 
 /**
@@ -396,6 +401,24 @@ public:
             NodeType, vlc_list, iterator, const_iterator
         >(l, node_ptr) {};
 
+    template <typename IteratorType>
+    IteratorType erase(IteratorType it)
+    {
+        vlc_list_remove(&((*it).*(this->_node_ptr)));
+        return it;
+    }
+
+    void push_front(NodeType &item)
+    {
+        struct vlc_list *node = &(item.*(this->_node_ptr));
+        vlc_list_prepend(node, &this->_list);
+    }
+
+    void push_back(NodeType &item)
+    {
+        struct vlc_list *node = &(item.*(this->_node_ptr));
+        vlc_list_append(node, &this->_list);
+    }
 };
 
 /**