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.
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.