trait for entity whis files

[ivan1986]

this trait fix #123 #128 #96

for call doctrine event listenert need change one or more field

this trait add set and get for all files fields and add del field (checkbox or false to file)

I know about it in not good decision, but it's work
pleace help impruve and integrate this trait

[ftassi]

As far as we don't merge #106 I think the best solution for this problem is #8 (comment)

I would accept a PR with a trait that contains the fix in #8 (comment) but I don't really feel the need for it, as we're talking about a few line of code to solve an issue that is well documented as a known issue.