The option for the player techCanOverridePoster is introduced in this commit. It allows techs to update the post whenever they like. isPosterFromTech_ is introduced as a private player field in order to track when a poster was set by a tech. This allows us to clear the poster whenever the tech is disposed of by the player.
Additionally, attempting to set the same poster more than once will have no effect / no changes will be made, since the poster is the same. This was done in order to stop triggering multiple posterchange events when calling player.poster(aPoster) with techCanOverridePoster set to true.
When a tech is disposed and a poster was set by it, unset the poster.
Pass a `canOverridePoster` option to techs to know whether techCanOverridePoster was set.
Fixes#4910.
A possessive belonging to an "it" doesn't need an apostrophe. Don't believe me? Ask [the Oatmeal](http://theoatmeal.com/comics/apostrophe) (look for the velociraptor)!
