[ENH] Separate canvas and base widget

[ales-erjavec]

Issue

Ref gh-3527 (Standalone canvas)

Separate canvas and base widget out of Orange3
Using:

https://github.com/biolab/orange-canvas-core/
https://github.com/biolab/orange-widget-base/

This pull request builds on top and supersedes gh-2792

Description of changes

• Of the canvas, only the main application entry point remains.

• The workflow model with the base widget is moved to orange-widget-base. This includes some but not all of gui.py, utilits/..., basically what is independent of Table, Variable, ... but the package structure is reconstructed here.

• Orange.data.io.FileFormat is no longer used to registers image writers Orange.data.io.FileFormat.img_writers returns empty sequence.

• The Orange.widgets.widget.OWWidget extends OWBaseWidget with Table,Domain,... related report methods.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #3772 into master will increase coverage by 0.4%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #3772     +/-   ##
========================================
+ Coverage    84.3%   84.7%   +0.4%     
========================================
  Files         384     371     -13     
  Lines       72755   64836   -7919     
========================================
- Hits        61334   54920   -6414     
+ Misses      11421    9916   -1505

[markotoplak]

Before the final split, should we clean up https://github.com/biolab/orange-canvas-core/ and
https://github.com/biolab/orange-widget-base/ so that they take less space? Filtering paths that are not needed there anymore should be relatively straightforward.

[ales-erjavec]

https://github.com/ales-erjavec/orange-widget-base/tree/master-filtered has filtered history

Using rewrite.sh

git checkout -b orange-canvas-core-rebase a41a4c4fa9d32d5d84b48aa4e1fb7984c2216cf2
git filter-branch -f --prune-empty --index-filter '
KEEP_LIST=(
    doc/development
    README.md
    .github
    setup.{py,cfg}
    MANIFEST.in
    LICENSE
    CONTRIBUTING.md
    scripts
    .github/
    .travis/
    .travis.yml
    appveyor.yml
    Orange/{__init__,util}.py
    Orange/widgets/{widget,settings,io,credentials,gui,__init__,widgetTemplate}.py
    Orange/widgets/{utils,tests,icons,report}
    Orange/canvas
    "./requirements*.txt"
    .gitignore
    .gitattributes
    pylintrc
    distribute
    .coveragerc
)

git rm --cache -r -q -- ./
git reset -q $GIT_COMMIT -- ${KEEP_LIST[@]}
' -- orange-canvas-core-rebase 

git cherry-pick a41a4c4fa9d32d5d84b48aa4e1fb7984c2216cf2..master

Another option would be https://git-scm.com/book/en/v2/Git-Tools-Replace and keep only the history from the split onward.

orange-canvas-core already has only pertinent history.

[ales-erjavec]

should we clean up https://github.com/biolab/orange-canvas-core/ and
https://github.com/biolab/orange-widget-base/ so that they take less space?

The https://github.com/biolab/orange-widget-base/ is now filtered