Fix crash at shutdown.

The context pointer was dereferenced after the context object was deleted. This is fixed by storing a pointer to the MainWindow in QTFullScreen, so context does not have to be dereferenced.

The logic in QTFullScreen::eventFilter is also simplified a bit.
Merge pull request #840 from ibooij/fix_crash_in_qtfullscreen

Fix crash at shutdown.
CP Chart Overlay

.. now has an overlay widget that can contain multiple widgets
   and is user movable and resizable etc

.. we show the model derived values for now, but could add other
   things at a later date (e.g. MU distribution etc).

Fixes #811.
Truncate W'bal going hugely negative

.. this is caused by bad data and/or the spline getting
   spoilt. We need a better way of decritizing the data.
Added some code comments

.. to GcSideBarItem.cpp as the code is very difficult to
   follow. Not hugely valuable comments but they will at least
   help to understand the purpose of each widget.
Add menu button to chart bar

.. code lost and reinstated, need to write the code
   to show the chart menu when it is clicked.

.. just pushing this code to get it back into the repo
   as it was lost accidentally with some other recent changes
Add Chart now back on Chart Bar

.. so at least chart adding should be easier and no more
   UX issues.

.. need to look to add other menu options there; add page, athlete,
   chart settings.
Some notes on what's new in GC 3.1
ShareDialog: Prepare for GarminConnect Upload
Rank
Twitter: api.twitter.com requires SSL/TLS for all connections
Smaller font on CP widget

.. 16 points was too big !
Twitter: use OAuthDialog for authorisation
Add Vector/Rotor TE and PS data series

.. Added the torque effectiveness and pedal smoothness
   data series to the RideFile structures.

.. The only file formats that support it at this stage
   are Fit and  GoldenCheetah JSON.

.. As more file formats support it we will add it here.

.. The charts/editor now need to be updated to support
   these new data series.
Merge branch 'master' of github.com:/GoldenCheetah/GoldenCheetah
Add Vector data series to ride plot

.. Torque Effectiveness and Pedal Smoothness now
   added to the ride plot.
Fix CPPlot SEGV

.. crashes on low power files (eg. power = 0.1w)
liboauth is optional
Add Averages for Pedal Data

.. pedal smoothness and torque effectiveness
Updated release notes