cleanup: (minor) changed some strings to symbols

This is for subwindow orientation.

	modified:   bufferslist.rb
	modified:   filebuffer.rb
	modified:   keymap.rb
	modified:   src/keymap.rb
cleanup: moved "full text replace" into TextBuffer class.

Trying to lighten-up the FileBuffer class.

	modified:   filebuffer.rb
feature: make backups toggle-able

Large files with long histories can be slow (a second or two to save).
This way, the backups can be turned off on the fly.

	modified:   editor.rb
	modified:   keymap.rb
cleanup: moved some methods from filebuffer to textbuffer

More migriation out of filebuffer

	modified:   filebuffer.rb
Working on compatibility with ruby1.8.6.

Needs more testing.  Some minor backports of methods are in
compatibility.rb.  The rest is minor changes, or rescues for situations
where 1.8.6 doesn't know what to do.

	new file:   src/compatibility.rb
	modified:   src/filebuffer.rb
	modified:   src/jedi.rb
	modified:   src/bufferslist.rb
	modified:   src/syntaxcolors.rb