[Improvement] Implement design theme grid on ui component

[simpleadm]

Description

Several versions ago backend grid widgets (\Magento\Backend\Block\Widget\Grid) were marked as
deprecated in favour of UI component implementation. This PR refactors design theme grid to use UI components.
Grid location: Content > Design > Themes

At least it will save time of third party developers who needs to extend themes grid :)

Manual testing scenarios

Before changes:

After changes:

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[VladimirZaets]

Hi @simpleadm , any updates?

[simpleadm]

Hi @VladimirZaets ,
what updates you are waiting for?

[VladimirZaets]

Hi @simpleadm , we have 2 functional tests that were failed with your changes. That's are Magento\Widget\Test\TestCase\CreateWidgetsEntityTest.test with data set "CreateWidgetEntityTestWithDifferentThemes_0"
and Magento\Widget\Test\TestCase\DeleteWidgetEntityTest.test with data set "DeleteWidgetEntityTestVariation1_0".
Can you look and fix them, please?
If you need some help or guide how to running functional tests locally, please let us know.

[VladimirZaets]

@magento-engcom-team give me new test instance

[magento-engcom-team]

Hi @VladimirZaets. Thank you for your request. I'm working on Magento instance for you

[magento-engcom-team]

Hi @VladimirZaets, here is your new Magento instance.
Admin access: https://pr-14470.engcom.dev.magento.com/admin
Login: admin Password: 123123q
Please make sure you are PR author or assignee to access the instance.

[simpleadm]

Hi @VladimirZaets ,
Please check updates.

[VladimirZaets]

Hi @simpleadm, please make some refactoring:

1. The collection shouldn't extend the ThemeCollection collection, it should only implement SearchResultInterface.
2. Use ThemeCollection to getting access to data inside implemented methods.

[simpleadm]

Hi @VladimirZaets ,
Please check new changes.

[VladimirZaets]

Hi @simpleadm, thanks, I took this PR in progress.

[VladimirZaets]

Hi @simpleadm. The grid rows are not clickable, consequently, edit page is not available too.
Please fix it. Thank

[simpleadm]

Hi @VladimirZaets ,
View action was added, please check.

[VladimirZaets]

Hi @simpleadm.
We tried to test your fix and found the next problem:
View action redirects to Dashboard when Add Secret Key to URLs=Yes

[magento-cicd2]

All committers have signed the CLA.

[simpleadm]

Hi @VladimirZaets ,
Url was fixed.

[magento-engcom-team]

Hi @simpleadm. Thank you for your contribution.
We will aim to release these changes as part of 2.3.0.
Please check the release notes for final confirmation.