Observer adjustable time axis

# Motivation
  
  When using the performance graphs in Observer for a long time
  period, information may be lost due to the narrow time frame in which
  data is kept and plotted.

# Proposed solution

  A simple settings dialog box was added where the user may enter the
  preferred length of the time axis in minutes and seconds. The
  performance graphs update their time axis accordingly.

  The interval of the time marker positioning adjusts to different
  time axis lengths so that a reasonable number of markers are being
  rendered.

  Finally, the labels of the time markers show either minutes only,
  seconds only, or both. This is decided based on the time marker
  intervals. This way, the labels always show enough information but no
  more.

# Possible problems

  If the time axis is very long, the current method of storing and
  rendering the data may become resource-heavy. An upper limit has been
  considered, but that is probably best left to the user to decide.

# Implementation details

  A settings field was added to the state record of the performance
  graph module. The settings field is a record with currently only one
  setting: graph_time, which is by default set to 60 (the previously
  hard-coded value). Each part of the code where the time axis length of
  60 was hard-coded was changed to use the current setting instead.

  The dialog box is set to trigger a callback function which reads a new
  value from text fields, calculates the new graph_time value and sends
  a message back to the module to update settings accordingly.

  The time marker code was rewritten to select an interval from a list
  in decreasing order until an interval is found which is short enough
  to show at least 5 time markers.