change of terminal invoke(from 'gnome-terminal' to 'xterm')

Description:

1) added Examples:
   - BJT_amplifier
   - RC
   - RC_pcb
   - RC_ac
   - nodalExample_plot

2) changed 'gnome-terminal' to 'xterm' in
`OSCAD/forntEnd/newProject.py`

3) OSCAD desktop icon `oscad.desktop`
   iccon location: /usr/share/icons/oscad.png

I have made the changes in OSCAD/ directory and also in OSCAD.tar.gz
archive inside OSCAD_installer/ directory