Keybindings for commands in source buffer

Also new variable pdf-navigate-source-buffer, and append file name
to *PDF-History* buffer.

Altered items:
pdf-navigate-key-cmd, pdf-navigate-mode-map,
pdf-history-minor-mode-map, pdf-navigate-source-buffer,
pdf-navigate-window-size, pdf-navigate
New variable: pdf-navigate-display-lines

For setting the number of lines displayed in the *PDF-History* buffer.

Altered items:
pdf-navigate-create-entries