Multiclient server apps

• It is possible now to create server apps that can be accessed by multiple users simultaneously. To this end, one should use arguments
  sessionVars, beforeLoad and afterLoad of the openPage functions. sessionVars defines local variables with their default values
  for each client, beforeLoad and afterLoad are callback functions that are called once for each new web page. Other than that, there are
  no differences between creating an rlc app locally or on a server.

• rlc has been restructured so that the entire app is now stored inside a single R6 object of class LCApp (see man pages for more details).

• Property axesTitlePos added for all the charts that have axes. This property allows to place axis labels above or below the x-axis
  (to the left or to the right from the y-axis) and at the end, in the middle or next to the start of the axis.

• Now scatter plots in canvas mode also have ticks that indicate presence of points outside of the current axes range.

• NAs are correctly processed.

• Bug with mispositioning a click in Firefox v.70 fixed.