Add 'Start time' column to game list

Now, users won't have to manually include the creation time in the game description.
Two notes:
 1. tr() returns a QString, i'd avoid the double cast (QString to std::string here, then QString::fromStdString later) and just return a QString here.
 2. Each "string" needs to be translated separately, so tr() should be used around "s ago", "m ago", " hr ago", "5+ hours ago". The way it's implemented currently would force us to translate every possible string, like "1s ago", "2s ago", "3s ago" etc..