Are quasars simply AGNs that are viewed from a particular angle? I found the picture below in an answer to the post "Was the Milky Way ever a quasar". The picture is published on a website belonging to "NASA/IPAC Extragalactic Database". In the answer that has 16 "thumbs up" it says:


  A quasar is simply an active galactic nucleus (AGN) that is viewed
  from a particular angle; see the picture below, in which quasars are
  labeled "QSO". This is really a remarkable figure because historically
  all of the names in the figure were thought to correspond to different
  types of objects, when really they all refer to the same thing!




There is another answer to the question "Difference between quasar and Active Galactic Nuclei?" where someone states that a quasars: 


  main property is that it's galaxy axis points towards the earth and
  this way we receive its light and radio signals. That's why they are
  one of the most energetic AGN.


This answer has gotten six thumbs up. However, there is no reference to any source backing up that claim so it seems like private opinion.


Does the picture above represent the mainstream view among astronomers today or is it more like a private opinion of someone?
According to Wikipedia, ten percent of all observed galaxies are classified as Seyfert galaxies. Would we perceive all of those as "quasars" if we happend to observe them from the right angle? (Maybe there is also a luminosity requirement)
According to Wikipedia Markarian 231 is the closest quasar at a distance of 581 million light years. Also according to Wikipedia, the Circinus Galaxy is the closest Seyfert galaxy, only 13 million light years away. Would we perceive there to be many quasars closer than 581 million light years away, if we just happend to view the active galaxy from the right angle?
Could someone please show me the scientific investigations proving that the galactical planes of galaxies perceived to host quasars always lies within a certain range of observational angles as viewed from the Earth? These kind of investigations should be available if the claim that whether or not an active galaxy is perceived to be a quasar is almost totally depending on observation angle has a real backing and is not just speculation.
There is total confusion in the answers you refer to.
Quasars were first defined because of their star-like appearance - quasi-stellar objects or QSOs.
The fact that they are Galaxy-sized objects with point-like appearances means they are very luminous, dominated by central nuclear emission and far away (as confirmd by their high redshifts).
It was later recognised that quasars are just the high-luminosity end of the AGN zoo.
Quasars can also be viewed at a variety of angles and have distinct properties as a result. So there are radio-quiet and radio-loud quasars, broad absorption line (BAL) quasars and optically violently variable (OVV) quasars. The distinction between these types is thought to be dominated by viewing angle, mass of the central black hole and the accretion rate. The nomenclature can also vary depending on what wavelength region you are talking about (radio, optical, X-ray etc.).
These objects have lower luminosity counterparts amongst AGNs - FR I and FR II AGNs, Seyfert I and IIs and blazars (or BL Lac objects) - again, whose differences are largely explained by viewing angle.
I don't think these days it is helpful to think of quasars and AGNs as different objects. They are actually similar and it is just a question of the scale of the accretion process going on at their centres that is different and covers a continuum rather than a bimodal distribution.