More styling changes
Minor commit: Added "All Cameras" preset
Major commit: Bug fixes, gapless playback, all cameras preset added.

* Gapless playback now enabled by default (to be used by a UI toggle in
the future)
* By default the timeline shows the last 24 hours of activity on all
cameras but the Start and End range selection boxes showed incorrect
timestamps - this has now been fixed.
* Overhaul of the way currently playing events are handled. The
currentevent variable has been nuked as it was no longer used anyway.
The currentevents array is no more either - it has been replaced by
window[currentevents + monitorId]. An event tracker for each camera
ensures that playback doesn't stutter when an event is scheduled to
start playing before the previous event has ended.
* imgError function enabled again so if there is a problem with the
camera source a placeholder image is displayed.
* All cameras preset link added. Code added to ensure that the "Choose
Cameras" and "Choose Preset" options don't conflict with each other.
* Remove old clearCurrentEvent() function as it was redundant anyway
* Removed old comments and debug code