Ubuntu – Unable to connect to wireless internet(wifi) through KDE plasma desktop

Click on NetworkManager icon in the tray. Press 'Manage Connections' Go to Wireless Tab Press 'Add...' button but choose 'Shared'

Everything is ready, I've connected to this connection from the network manager but I dont know if that is important.

TL;DR there was a permission issue with ~/.cache/upstart/dbus-session, rm -rf .cache/upstart and a reboot solved it.

Newbie debug insights for future readers (which may well include me).

The ~/.xsession message I mentioned in the question, along the lines of

init: dbus pre-start process (PID XXXX) terminated with status 2

was actually very important. I learnt what it meant while investigating, I found it this way

• First, I thought that the issue was with kde, so I tried to find how lightdm started kde
• I tried to look into /etc/lightdm/ (as suggested by man lightdm) but the lightdm config files were not here but (I found it by looking into /var/log/lightdm.log) in /usr/share/lightdm/lightdm.conf.d where I found the relevant file /usr/share/lightdm/lightdm.conf.d/40-kde-plasma.conf
• According to it, what lightdm started for kde was the script /usr/bin/startkde, so I added some debug echo 'startkde is at line ##' > /home/evpok/delog lines to it to see where it failed, but none of them were executed.
• checking again /var/log/lightdm.log I saw the line
  Running command /usr/sbin/lightdm-session /usr/bin/startkde
  so I looked into usr/sbin/lightdm-session, to which I again added debugging echo lines
• After some fumbling, I found that the issue was with loading an Xsession script: /etc/X11/Xsession.d/99x11-common_start. So I looked into that one. It seemed to load normally and had only one line exec $STARTUP. Adding an echo to see what was in $STARTUP I found it was init --user. Now I didn't want to mess with init so I just traced its outputs by commenting this line out and adding instead
  exec init -v --user > /home/evpok/initlog 2> /home/evpok/initerrlog

• Looking into these logs, I saw this message in initerrlog

  dbus pre-start process (PID XXXX) terminated with status 2
  but I still didn't know what to make of it, so I looked into initlog and saw
  Loading configuration from /usr/share/upstart/sessions

• I looked into that dir, where I saw a startkde.conf. After some looking into documentation for upstart job confs, I saw that it had
  start on started dbus and xsession SESSION=kde-plasma
  obviously that was why startkde didn't start. It needed dbus, which had an error, so I looked into dbus.conf
• There, there was a pre-start script stanza, hey! That's what's in the errlog of init and in .xsession-errors. So for I added again some echoes, to find that the issue was at the line
  echo "DBUS_SESSION_BUS_ADDRESS=${DBUS_SESSION_BUS_ADDRESS}" >$HOME/.cache/upstart/dbus-session (Not one of my debug echoes)
• After some fumbling, I found that I couldn't touch the non-existent file ~/.cache/upstart/debus-session because of a permission issue. I tried creating it under sudo and chown it to me with appropriate permissions but it didn't work. So I just rm -rf'ed .cache/upstart.
• After a reboot, everything worked as expected.

After all I just needed to stop being afraid of the internals and getting my hands dirty.