libview: Initial implementation of caret navigation

Navigation by character, word, next/previous line and beginning/end of
the line using the caret cursor.

The routines to move the cursor don't use GtkTextBuffer to
avoid the duplication of the text for every page.
  - Left/right arrow: Move one character to the left/right.
  - Up/down arrow: Move up/down one line.
  - Ctrl + left/right arrow: Move to the beginning/end of
    the previous/next word.
  - Home/End: Move to the beginning/end of the current line.

https://bugzilla.gnome.org/show_bug.cgi?id=638905