option to not numberize maps, do not write useless blank line at map beginning

First commit adds an option to not write annoying numbers comments in map files like:

// entity 0
// brush 249

Option is opt-out and findable in Preference/Settings. Well, the option is to write number comments and is enabled by default, since it's the legacy behavior, and people can disable it:

This option is very useful for people tracking map changes in git repositories.

Second commit just rewrite a string with a better wording (“load last map at startup” instead of “load last map on open” because this option loads last map at NetRadiant startup, not on another map load…

Third commit makes NetRadiant not writing a useless blank line at beginning of map file any more.