AdminLTE 3 implementation (without gii)

[Tarinu]

This introduces some BC changes:

• Namespace got changed to dmstr\adminlte since dmstr\widgets was being overwritten by the dmstr\yii2-db package in testing environment. If there's a better solution for this, then I can change it back.
• iconClassPrefix in Menu widget was split into 2. FontAwsome 5 introduced different icon types (far, fas etc), so i thought it would be easier to be able to just change the type separately. The default type is fas (solid)
  It's possible to change the type with ['label' => 'Foo', 'iconType' => 'far']
• Header is just ['label' => 'Foo', 'header' => true] instead of ['label' => 'Foo', 'options' => ['class' => 'header']]
• Label is now badge and is set with the badge option ['label' => 'Foo', 'badge' => '<span class="badge badge-info right">123</span>']
• Then there's obviously the AdminLTE changes themselves. Like using bootstrap 4 instead of 3 etc

I left the Gii part out since I'm not really that familiar with how it's working.

#183

[schmunk42]

Looks great! Thank you very much.

@eluhr Please have a look a this PR.

[takizo]

Great work. Thanks a lot 🙏