Minor commit: Ensuring if many cameras are on screen you can scroll down
onerror function added to monitor stream <img> tag
activity now only generated by AJAX - means cleaner HTML sourcecode \0/
Bug Fix - If the user hasn't set a default preset "All Cameras" is chosen
Bug fixes. Code cleanup. Live view / playback mode fixes. Function added.

* Bug fix: Fixed an issue where going from Playback mode to Live View
would display no cameras. This was due to me trying to add cameras that
were already in "chosencameras"
* Bug fix: When adding a camera liveview would automatically be set to
true which was wrong
* Live view no longer set to true when a camera is added as you could
add cameras in playback mode
* Now when adding cameras in playback mode a placeholder image is shown
instead of the livestream
* When adding a camera the mode is now checked before requerying the
timeline
* loadUserDefaultPreset() function now exists which is called on page
load an when the user switches back to live view
* Old variables that were used to initially populate the timeline on
page load using data from playback.js.php are commented out - will be
removed in the next commit
* resumeLiveView() function removed as it's no longer needed
* Timeline now loads without the current time being shown, this is only
visible in playback mode now
* Bug fix: Fixed an error where showing all cameras would also try and
load the cameras from a preset
* Bug fix: When going from Playback mode to Live View the live streams
would show an error image - this was due to calling window.stop() which
was present due to the old method of displaying livestreams
* Bug fix: When entering live view liveview was set to false and vice
versa, this has been flipped
* Timeline current time playhead and custom time playheads now
shown/hidden at the right times (hopefully)
* Before checking if any events need to be played, a check on liveview
mode is done