core: add identifier in buffer lines (closes #901)

core: add identifier in buffer lines (closes #901)

For buffers with formatted content, the "id" starts to 0 on each buffer and is
incremented on each new line displayed (it is reset to 0 if reaching INT_MAX).

For buffers with free content, the "id" is set to the same value as "y" (ie the
line number, starting to 0).