[cooja] Set location of newly created plugins relative to second last activated plugin.

Setting the position based on the number
of total inner frames was very inflexible
and caused pad positioning for larger number
of frames.

Setting the location of new plugin frames
to the second last activated one tries to
meet two requirements:
- Avoid covering the last activated plugin frame,
i.e. the one that was active when new plugin
start was invoked.
- Set new plugin near an actively used desktop pane
location and allow 'diagonal stacking'
Merge pull request #639 from ejoerns/pull-req/cooja-plugin-pos

[Cooja] Set location of newly created plugins relative to second last activated plugin