fix for bug #839 + fix for the "impossibility to insert tab" bug

[Alexey Sintutin]

Hello!

I noticed that's impossible to insert tab using top ruler.
Here is a patch for it + fix for bug #839 + bounding of Left Ident Mark,
Right Ident Mark and Hanging Ident.
I will write First Line Mark bounding code tomorrow.

Bye, Alexey.

839.patch