event queue pruning

[msmeissn]

if we received an objectadded event and later a objectremoved event, the internal event code could remove the objectadded if it was not yet fetched by the external user of the library.

this avoids a ptp chdk issue ( https://gist.github.com/anonymous/8f0ed5effc88173c0ea0 ) where we try to getobjectinfo on a already deleted image, because our event handling is too late.