Merge bitcoin-core/gui#116: Fix unreasonable default size of the main window without loaded wallets

7b2e42ecc4bddb5504665d6932dc5ffdbd3b593e qt: Add WalletFrame::sizeHint (Hennadii Stepanov)

Pull request description:

  This PR fixes a bug in master (d67883d01e507dd22d1281f4a4860e79d6a46a47) and in 0.20.1 that could be easily reproduced with
  ```
  $ src/qt/bitcoin-qt -regtest -resetguisettings -nowallet
  ```

  ![Screenshot from 2020-10-25 21-21-27](https://user-images.githubusercontent.com/32963518/97117179-b1800100-170a-11eb-87c9-3120d39b9455.png)
  ![Screenshot from 2020-10-25 21-23-32](https://user-images.githubusercontent.com/32963518/97117186-b644b500-170a-11eb-8b5d-234ff7205003.png)

  **With this PR:**

  ![Screenshot from 2020-10-25 21-20-35](https://user-images.githubusercontent.com/32963518/97117226-f441d900-170a-11eb-8d66-98b7718a2bb1.png)
  ![Screenshot from 2020-10-25 21-23-03](https://user-images.githubusercontent.com/32963518/97117232-f99f2380-170a-11eb-85ed-c7b5ece926b2.png)

  ---

  Fix #104
  Fix #113

  This PR is an alternative to #107 without [hard-coding a size in pixels](https://github.com/bitcoin-core/gui/pull/107#discussion_r511474021).

ACKs for top commit:
  jonasschnelli:
    Tested ACK 7b2e42ecc4bddb5504665d6932dc5ffdbd3b593e - I can confirm this fixes #104 (Ubuntu 20.04 - HiDPI 200%).

Tree-SHA512: eb0692dbeb3befdeecca0e41534c9783eab6637c14cc4f170ee42619235884f9354f8d22a10c20c08cc89dc5340a60b7dfa2523c12e64b3386b3fd2c6d5f934e

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