Add language setting and translation function

This adds a language selector to the settings dialog. It will allow us to prepare multilingual distributions of Orange, which we urgently need for various education-related projects.

The change is only visible if distribution includes message files that are prepared by (so far unpublished new version of) Trubar.

Caveats:

Language must be detected at the very beginning because some modules (even modules like Orange.data.table may already include translated strings). This is why language is read from QSettings(QSettings.IniFormat, QSettings.UserScope, organization, application) and not just QSettings() because QApplication is not set up at that point yet. Organization and application are specified in Trubar's configuration files, to allow Trubar to insert the following at the beginning of each module (after imports from future):
```
from orangecanvas.utils.localization import Translator  # pylint: disable=wrong-import-order
_tr = Translator("orangecontrib.prototypes", "biolab.si", "Orange")
del Translator
```
After changing the language, Orange must be re-run with --force-discovery. This belongs to this PR, but is not implemented yet.

@ales-erjavec, do you have any comments?

To do:

[x] Restart with force-discovery, or a message that changing this setting will take effect after restarting, and then at the restart compare the previous and the new language and trigger force-discovery if needed.
[x] Use system locale to set default language
[ ] In message files, add language name in English and in native language; also provide place for any other info we may want to add in the future.

I'm attaching examples of core files and some add-ons. Strings are simply looked up in a table, e.g.

        gui.button(box, self, "Save", callback=self.save)
        gui.button(box, self, "Load", callback=self.load)
        gui.button(box, self, "Reset", callback=self.reset)

becomes

        gui.button(box, self, _tr.m[233], callback=self.save)
        gui.button(box, self, _tr.m[234], callback=self.load)
        gui.button(box, self, _tr.m[235], callback=self.reset)

while f-strings, like

            tpe = f"Random sample with {self.sampleSizePercentage} % of data"

are compiled and evaluated,

            tpe = _tr.e(_tr.c(573))

m, c and e are attributes of Translator class from this PR.

orange3.zip orange-canvas-core.zip orange-widget-base.zip orange3-imageanalytics.zip orange3-geo.zip orange3-network.zip

Preparation of these files is not a part of this PR.

Codecov Report

Attention: Patch coverage is 56.36364% with 24 lines in your changes are missing coverage. Please review.

Project coverage is 76.61%. Comparing base (8a3ecf1) to head (352808d).

Files	Patch %	Lines
orangecanvas/utils/localization/__init__.py	64.86%	13 Missing :warning:
orangecanvas/application/settings.py	23.07%	10 Missing :warning:
orangecanvas/main.py	80.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #296 +/- ## ========================================== - Coverage 76.67% 76.61% -0.06% ========================================== Files 99 99 Lines 21104 21158 +54 ========================================== + Hits 16181 16210 +29 - Misses 4923 4948 +25 ```

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biolab / orange-canvas-core

Add language setting and translation function #296

To do:

Codecov Report