Feature/lookup field

[ibelar]

Implementation of Lookup field (see ticket #550 ). Simple use:

$form = $app->add(new \atk4\ui\Form(['segment']));
$form->add(['Label', 'Add city', 'top attached'], 'AboveFields');

$l = $form->addField('city',['Lookup']);

// will restraint possible city value in droddown base on country and/or language.
$l->addFilter('country', 'Country');
$l->addFilter('language', 'Lang');

//make sure country and language belong to your model.
$l->setModel(new City($db));

REVIEW TODO:

• need a demo (possibly including some more SQL data)
• filters should reload each-other and only show possible options
• documentation missing
• code coverage is needed

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (develop@3aa52ef). Click here to learn what that means.
The diff coverage is 1.84%.

@@            Coverage Diff             @@
##             develop     #551   +/-   ##
==========================================
  Coverage           ?   63.57%           
  Complexity         ?     1690           
==========================================
  Files              ?       92           
  Lines              ?     4082           
  Branches           ?        0           
==========================================
  Hits               ?     2595           
  Misses             ?     1487           
  Partials           ?        0

Impacted Files	Coverage Δ	Complexity Δ
src/FormField/Lookup.php	0% <0%> (ø)	49 <49> (?)
src/FormField/Generic.php	95.45% <100%> (ø)	11 <1> (?)
src/FormLayout/Generic.php	97.4% <100%> (ø)	34 <0> (?)

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[romaninsh]

is this ready for review now?