Kickoff redesign

This commit completely overhauls Kickoff's user interface!

Original design (UI/UX) discussion
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- https://phabricator.kde.org/T12192
- !186

Features / issues
-----------------
New:
- Double pane UI
- Both keyboard and mouse navigation have been overhauled
- Power/Session actions are available in the footer
- Improved RTL support
- Grid view
- All applications view
- Accessibility

Screenshots / Videos
--------------------
Apps

![image](/uploads/b44c0e36d7687b22f9df563ed19aad19/image.png)

Places

![image](/uploads/93ad74bd8e037dc70d4927eb20a8ec29/image.png)

Search

![image](/uploads/f77da9df55bd1f9e75c0b0de96eebfd8/image.png)

Accessibility

https://youtu.be/Vqah3yL5h_E

Bugs fixed
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BUG: 357379

BUG: 369159

BUG: 369405

BUG: 378815

BUG: 387510

BUG: 387797

BUG: 391383

BUG: 392005

BUG: 393401

BUG: 394924

BUG: 398234

BUG: 398279

BUG: 398349

BUG: 398350

BUG: 398352

BUG: 398353

BUG: 405674

BUG: 408313

BUG: 410018

BUG: 412090

BUG: 415392

BUG: 416423

BUG: 419169

BUG: 423160

BUG: 425970

BUG: 426329

BUG: 427846

BUG: 428297

FIXED-IN: 5.21

@teams/vdg