additonal timelines and next/previous keyboard shortcut for markers

After using blender with some scripts to test out its video editing I found that Kdenlive could really
use some keyboard shorcuts to jump between the next and previous markers. This way one could setup their
clips with markers for example at every new take of a shot and just jump back and fort between them quickly
when deciding which take to use.

A way to handle multiple timelines in the same session would be great as well so we could have one window open
and manage different edits for the same project with the ability to drag back and fort between them.

Just a few ideas.