Use QT file dialogs on Linux

.. cause native are borked on Ubuntu.
Speed up sidebar painting

.. by setting the linear gradient up in advance rather than
on every repaint.
Fix Histogram window custom date selection

.. it wasn't updated properly when fixing custom
date ranges in LTM.

Fixes #509.
Reinstate mini calendar on sidebar

This is the first step to reinstating the calendar on the analysis view
sidebar. Next step is to enhance it to show multiple months and then to
refactor the diary sidebar GcCalendar to use it.
More Linux file dialog workarounds

.. for export activity and export metrics.

Fixes #504. (again, it was reopened).
UI Nits: Splitter handle show/hide button gone

.. taking out last remnants in the code from previous design. This will
also remove the focus block that appears on calendar handle in diary view,
but only really visible on Linux with Gtk/Gnome.
make Zones class more generic, and QFile argument for write() like it is on read(), preparing 'Critical Speed' zones for running
Merge branch 'master' of https://github.com/GoldenCheetah/GoldenCheetah
Calendar sidebars now show multiple months

.. if you resize them they will show as many months
as they can fit vertically.

.. also removed the scrollbars in the summarys shown
in the diary and LTM sidebar

Fixes #503.
Merge branch 'master' of https://github.com/GoldenCheetah/GoldenCheetah
Remove big numbers at top of Diary calendar

Especially since the date selected is almost irrelevant on the
diary sidebar anyway!
Fix LTM group by selection

Fixes #499.
Merge branch 'master' of https://github.com/GoldenCheetah/GoldenCheetah
Fix JSON reader ignoring open fail

D'oh.

The ridefile reader for JSON files spotted when a file failed
to open but then continued to try and parse it!  Now returns NULL
when fopen fails.

This is particularly relevant when using foreign characters in
a filename passed to fopen(). We need to remove the fopen stuff
and replace if QFile reading to a buffer.
Replace fopen() in JSON ride file reader

.. since it fails to open files with foreign characters in the
path name. We now read via QFile into a buffer.
Merge branch 'master' of https://github.com/GoldenCheetah/GoldenCheetah
GcLabel paint is a CPU hog

.. and for no good reason.
Windows Look and Feel

.. akin to the Mac look and feel
.. still needs a bit of cosmetic work, but basically sound
.. also added in fixes to native file imports

Fixes #404
Fixes #504
Windows Look and Feel Mac Fixups

.. fixes the Mac build
.. Windows still crashes
Fix Windows Look and Feel crash on ICAL

.. fixed crash when compiling without ICAL
.. I found because my Windows build doesn't have ICAL