Various improvements to calendar widget

[matwachich]

Description:

Some enhancements to Calendar widget and DateEntry

• added localized month name
• added a function to programmatically change the currently displayed date
• added the ability to select a date, and a function to programmatically change the selected date
• fixed crash when OnChanged callback in not set
• calendar of DateEntry will now open with currently entered date selected
• corrected Calendar popup positioning (do not resize the popup, keep it always at its minSize)
• added a localized button under the Calendar in DateEntry to select today's date

What to discuss next:

• adding a config variable WeekStart to Calendar to override configured first Week day
• adding something similar for date format to DateEntry (did not do it, will have to study it)
• discussing is it judicious and idiomatic that tapping the selected date in the calendar will unselect it (setting SelectedDate = time.Time{})

Another one I just want to understand:

• Why using in DateEntry a *time.Time and not a time.Time ??? We can just set Date = time.Time{} (Zero-date) instead on nil

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.

[coveralls]

coverage: 62.21% (-0.1%) from 62.312%
when pulling 4442950 on matwachich:calendar_additions
into 0c929c1 on fyne-io:develop.

[Jacalz]

Thanks for this. Though, I would like to point out that PRs that introduce new bigger features are usually easier to review and land if they focus on one at a time. Fixes and smaller changes can often be grouped together though. Not something you need to change now but it is worth keeping in mind m

[Jacalz]

Sorry for the late review. I added some minor comments inline. It would also be good to add some tests for this. Also, please do tick the boxes in the PR template as appropriately.

[matwachich]

Ok done for the inline comments.
For the tests, I have to admit that... I never wrote tests for fyne, nor for golang...
I vaguely know the principle, but have no clue how to really write usefull tests...
If someone could help about this :)