Merge bitcoin-core/gui#391: Add cancel button to configuration options popup

0b869df1c913839855148d728514c76ba7664092 qt: Add cancel button to configuration options popup (Shashwat)

Pull request description:

  This PR renames the **OK** button to **Continue** and adds a **Cancel** button to the configuration options pop-up.

  This feature will give the user an option to abort opening the configuration file if they want to. This is an essential helpful feature that was missing in the master branch.

  In some windows managers such as Windows I3. The exit button at the top right corner is missing. So this feature becomes crucial there. And even when the exit button is there, it doesn't prevent the opening of the configuration file even when pressed.

  Additionally, it will always be possible to close using Keyboard Shortcut. This PR helps accessibility for those who need to use a mouse.

  <table>
    <tr>
     <td>Master
     </td>
     <td>PR
     </td>
    </tr>
    <tr>
     <td>

  ![Cancel-conf master(1)](https://user-images.githubusercontent.com/85434418/127555137-7a16dffd-109d-4024-917b-6b85f4df4f4a.png)

     </td>
     <td>

  ![Screenshot from 2021-09-07 20-15-28](https://user-images.githubusercontent.com/85434418/132365729-14f71f92-220b-4bb6-bed4-8315bd5697e6.png)

     </td>
    </tr>
  </table>

ACKs for top commit:
  hebasto:
    ACK 0b869df1c913839855148d728514c76ba7664092, tested on Linux Mint 20.2 (Qt 5.12.8):
  prayank23:
    tACK https://github.com/bitcoin-core/gui/pull/391/commits/0b869df1c913839855148d728514c76ba7664092

Tree-SHA512: c314e8b84064134f028f66f5015eb0f6ba33d5d4174c9ff49dcb5d2b577dce6019f59f9c7913393a415a323ea98c26febf5ca26e3e2102e7a1d31171e01937f1

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