Add master checkbox to Grid

[mhuser]

fix #1877

Add a master checkbox into grid header that enables to toggle selection of whole visible records.

[mhuser]

I have a first working master checkbox but the code is ugly and it does not account for the indeterminate state if child checkboxes are changed.
I found this fomentic example
Would it be cleaner to use a separated javascript file?

[mvorisek]

needs a Behat test

master checkbox must reflect the child states - unchecked/"partially checked"/"all checked"

if Fomantic-UI/jQuery cannot handle this in a few lines, the JS should be put into js/, but until you have fully working solution, feel free to keep the code uncompiled

[mhuser]

Ok, it works as of now with the indeterminate state if the child checkboxes are not all in the same state.

I had some issues to have the javascript statements to appear at the proper position. I first wrote them in Grid->addSelection() and they were called kind of before the checkbox() creation for fomantic-ui.

Still I feel the manner I use the javascript is not optimal because it does not look like other atk4/ui sources.

[mhuser]

I did not find any figures of the checkbox column inside the Column/Checkbox or the Grid documentation.

There seem to be no documentation updates to do because the core functionalities of Grid and Column/Checkbox are not affected.

[mvorisek]

@mhuser, thank for this great contribution and sorry for longer review! ❤

[mhuser]

@mvorisek
You are welcome. This is great! 😃