Wish: provide a way to run kde 4 with vivid

After an update from kubuntu utopic to vivid, I find the plasma 5 desktop to be too immature for production use.

- Plasma frequently crashes
- Some widgets once added cannot be removed because they make plasma crash when one tries to remove them (notably cpu load)
- Many widgets that used to exist in plasma 4 are missing (notably: a sensors widget, a cpu load widget that can be placed in a panel, a scientific calculator widget)
- The login manager does not seem to support a guest user
- The system settings are incomplete wrt what was available in plasma 4
- Screen savers are missing
- Some buttons in system settings do nothing (e.g. Online Accounts -> create -> Google). Why are those buttons there in first place?
- Plasma desktop crashes when adding an external screen and switching primary output
- The session manager is not able to restore applications to the proper virtual desktop
- Kwin fails badly on nvidia hardware with legacy 304 driver
- Hplip cannot dock in the system tray

These were found by a single person in just 3 hours of use. I am sure some more hours will find more.

Trying kde 5.3 from the ppa did not help.

Individual bugs have been duly been reported and in some cases, links to patches and workarounds have been provided.

But still... software that is so immature, with so many regressions and missing functionalities with respect to previous version should not be shipped as a default and - most important - as the single option. We have already seen it with the kde 3 -> kde 4 transition.

The idea that one has the option, because it is possible to stay with kde 4 by using the LTS version of kubuntu is wrong: if one has already updated the system from trusty to utopic, there is no easy way to downgrade from utopic to trusty LTS.