Reverts commits 932ece and 59faf. The side effect of this hack was that
Orange3.app did not display any icons.

setup.py: Tolerate missing numpy until build_ext is actually invoked

Make it possible to invoke setup.py and retrive basic package metadata without
numpy installed. This makes it possible to pip install into an
environment without numpy preinstalled:

    https://pip.pypa.io/en/stable/reference/pip_install/#installation-order

[FIX] Tests: Relative test datasets search path

[FIX] Fix a crash when saving report to a .report file

[FIX ] Update Classification Tree tutorial

Allow test to run with any cwd not just from the source root dir

Pickling widgets is prone to errors, as some components might not be
picklable. When such objects are unpickled, __init__ is not called,
causing further problems (such as not being able to pickle the object
again).

Fix Qt plugin paths (Refs: 59faf293)

site.getsitepackages() doesn't work inside virtualenv:
https://github.com/pypa/virtualenv/issues/228
https://github.com/pypa/virtualenv/issues/355

Manually specify plugins path

Fixups

[ENH] Predictions: Report, Data <--> Predictions, fixups

[FIX] OWSilhouettePlot: Fixup

The widget crashed when passed dataset (with metas) where an arbitrary class value belonged to only one data instance.

Win installer: upgrade pip and scikit-learn in the windows application installer

Linear projection: Improve GUI

[ENH] Add auto-apply checkboxes to learner dialogs

pip wheel is used to retrieve all package dependencies, and should
also consider .whl files build with wheel>=0.27

Require pip 8.* in order to install .whl files created by wheel>=0.27
Keep the wheel==0.26.* in the build env just to be sure.

[ENH] Report for Edit Domain widget.