Refactoring of save_index.


New class savegame::save_index_class with instance save_index_manager:
Methods:
       * data() - old save_index::load() - load save_index from disk if doesn't
loaded yet
       * data(const std::string& name) - get (or create if not available) summary
from save_index (from data())
       * write_save_index() - write current save_index to disk
       * rebuild(const std::string& name, const time_t& modified) - re-load summary
from savefile, update summary, write updated index to disk
       * rebuild(const std::string& name) - rebuild where modified == file
modification time
       * set_modified(const std::string& name) - remember modified time of file
       * get(const std::string& name) - check data(name)["mod_time"] - if different
from remembered by "set_modified" - rebuild


Class savegame::save_info changed
       *  time_t and std::string members now private. You should use method
for access to it
       * summary() method for transparent access to summary
(save_index_manager.get(name()) - see save_index_manager.get description
above)



Class savegame::manager removed. All statis function moved to savegame
namespace.



Function "savegame::extract_summary_data_from_save" removed, because it
word-by-word equalient to "extract_summary_from_config". I just call "rebuild"
for save_info in "finish_save_game". My purpose was avoid copy-paste.

Other changes:
       * I removed summaries from src/dialogs.cpp. Now you can transparent access by
save_info::summary()
       * I removed all copy-paste like (use summary() instead of this):
--------------------------------------------------
       std::string dummy;
       const config &summary = *summaries_[index_];
       if (summary["label"].empty())
       {
               try {
                       savegame::manager::load_summary((*info_)[index_].name, *summaries_[index_],
&dummy);
               } catch(game::load_game_failed&) {
                       (*summaries_[index_])["corrupt"] = true;
               }
       }
--------------------------------------------------
       * added class filename_filter - predicate for erase (used in get_saves_list)
       * added class create_save_info - builder for save_info. In general you can't
create save_info right now (and this is unecessary). If you need this in
feature - just move declaration of class create_save_info to header.