Do not open multiple pdf files in the same window, but open
a new window for each file opened (there is no way to
display multiple open pdf files side-by-side otherwise).
Added hjkl mappings to scroll within pages. Added movement
between highlights with `]h/[h`, although I am not sure it
works in both directions with these mappings.
Since sioyek does not support including into its configuration
file, we switch it to dotter templating and use the in-place
flavours insertion on theme selection for theming.
Any theming will not back filter into the repository.
