Add default values for ExternalEditor

As a script writer I find it annoying that I have to set up an editor for the viewer when my operating system already knows what editor is for what file types. It should - by default - open whatever my operating system thinks is the editor for LSL files.

While some users may want a different editor that what their operating system recommends, that can still be accomplished - by either changing the settings in the operating system, or by changing the ExternalEditor setting in the viewer.

To accomplish this I propose the following values for ExternalEditor:

• For Mac OSX: /usr/bin/open "%s"
  • Tested on Snow Leopard and Lion. I expect that all useful past, and predictable future, editions should work the same.
• Windows: C:\Windows\System32\cmd.exe /C START "%s"
  • Tested on Win7. I expect that all useful past, and predictable future, editions should work the same.
• Linux: /usr/bin/xdg-open "%s"
  • This is the hard one. Not sure how widespread xdg-open is, but it was there on my system. Untested, but should be viable as a generic default according to to my research.

These proposed values have been working well for me. All my editors (TextWrangler, Programmer's Notepad, Kate) either already support syntax highlighting for LSL or have free/easy-to-make modules for adding such.