Add trailer button to MoviePrePlay templete

Looks up trailer using Youtube if trailer button is pushed and loads
trailer using the YouTube channel.
Changed trailer query source to themovidedb

- Added some error messages (and a mirror of the python XML_Error
function in javascript)

Current bug: Some trailers are found but do not play and result in no
action (no error, no video, no loading, etc). I am still tracking this
down.
Fixed trailer lookup for movies with quotation marks

- Added ATTRIB_SVAL for "safe" values (escaped)
half-way turn back of 2d4dc9501a as CUTting a disabled COPY doesn't make much sense…
Merge pull request #172 from salbahra/master

Add trailer button to MoviePrePlay template
re-add key to timeline calls, now with key from PMS to correctly handle external stuff (channels) as well.
addon to 13817551eb - remove sessionstorage[ ] from PrePlayScreens, too.
Merge branch 'master' into personal-mods