Fix invalid/unnecessary string escape sequences

[gkatev]

This fixes a couple of occurences of invalid/unnecessary escape sequences in strings. In Python 3.12 these result in SyntaxWarnings. The one in StartPage is what we're most interested in, as it appears quite prominently when opening FreeCAD:

Error log:

$ FreeCAD                                                                                        1 ↵
FreeCAD 0.21.0, Libs: 0.21.0R38998 (Git)
© Juergen Riegel, Werner Mayer, Yorik van Havre and others 2001-2023
FreeCAD is free and open-source software licensed under the terms of LGPL2+ license.
FreeCAD wouldn't be possible without FreeCAD community.
  #####                 ####  ###   ####  
  #                    #      # #   #   # 
  #     ##  #### ####  #     #   #  #   # 
  ####  # # #  # #  #  #     #####  #   # 
  #     #   #### ####  #    #     # #   # 
  #     #   #    #     #    #     # #   #  ##  ##  ##
  #     #   #### ####   ### #     # ####   ##  ##  ##

/usr/lib/freecad/Ext/freecad/fc_cadquery/selectors.py:178: SyntaxWarning: invalid escape sequence '\|'
  """
/usr/lib/freecad/Ext/freecad/fc_cadquery/selectors.py:556: SyntaxWarning: invalid escape sequence '\|'
  """
/usr/lib/freecad/Ext/freecad/fc_cadquery/selectors.py:608: SyntaxWarning: invalid escape sequence '\|'
  """
/usr/lib/freecad/Mod/Start/StartPage/StartPage.py:151: SyntaxWarning: invalid escape sequence '\/'
  _Re_Pattern = "<Property name=\"{}\".*?String value=\"(.*?)\"\/>"

[realthunder]

Thanks a lot!