Clean up pointer use:

* Use unique_ptr for Actions instead of manual memory management
 * Use unique_ptr in KeywordDatabase
 * Use unique_ptr instead of shared_ptr for file formats
 * Don't pass shared_ptr to Reader/Writer, use references instead
Also
 * Switch to C++17 so we can use map::try_emplace

src/data/keyword.hpp

@@ -140,7 +140,7 @@ DECLARE_DYNAMIC_ARG(KeywordUsageStatistics*, keyword_usage_statistics);
  *  The database should be rebuild.
  */
 class KeywordDatabase {
-  public:
+public:
   KeywordDatabase();
   ~KeywordDatabase();
   
@@ -165,8 +165,8 @@ class KeywordDatabase {
    */
   String expand(const String& text, const ScriptValueP& match_condition, const ScriptValueP& expand_default, const ScriptValueP& combine_script, Context& ctx) const;
   
-  private:
-  KeywordTrie* root;  ///< Data structure for finding keywords
+private:
+  unique_ptr<KeywordTrie> root; ///< Data structure for finding keywords
   
   /// (try to) expand a single keyword
   /** If the keyword matches: