5a4a15d qt, refactor: Drop no longer used PeerTableModel::getRowByNodeId func (Hennadii Stepanov)
9a9f180 qt, refactor: Drop no longer used PeerTableModel::sort function (Hennadii Stepanov)
778a64a qt: Use PeerTableSortProxy for sorting peer table (Hennadii Stepanov)
df2d165 qt: Add peertablesortproxy module (Hennadii Stepanov)

Pull request description:

  The "Peers" table in the "Node" window does not hold multiple selection after sorting.

  This PR introduces a `QSortFilterProxyModel` subclass, that is a standard Qt [practice](https://doc.qt.io/qt-5/model-view-programming.html#custom-sorting-models) for such cases.

  Now the sorting code is encapsulated into the dedicated Qt class, and we do not need to maintain it.

  Fixes dashpay#283 (additionally).

  ---

  On **master** (7ae86b3):
  - rows are sorted by "Ping", and a selection is made
  ![Screenshot from 2020-11-28 22-53-11](https://user-images.githubusercontent.com/32963518/100525900-96eaed00-31cc-11eb-86e7-72ede3b8b33c.png)

  - rows are sorted by "NodeId", and the previous selection is _lost_
  ![Screenshot from 2020-11-28 22-53-21](https://user-images.githubusercontent.com/32963518/100525904-9c483780-31cc-11eb-957c-06f53d7d31ab.png)

  With **this PR**:
  - rows are sorted by "Ping", and a selection is made
  ![Screenshot from 2020-11-28 22-39-41](https://user-images.githubusercontent.com/32963518/100525776-06aca800-31cc-11eb-8c4e-9c6566fe80fe.png)

  - rows are sorted by "NodeId", and the row are still selected
  ![Screenshot from 2020-11-28 22-39-53](https://user-images.githubusercontent.com/32963518/100525791-2348e000-31cc-11eb-8b78-716a5551d7ec.png)

ACKs for top commit:
  jarolrod:
    re-ACK 5a4a15d, tested on macOS 11.2 Qt 5.15.2 after rebase
  promag:
    Tested ACK 5a4a15d.

Tree-SHA512: f81c1385892fbf1a46ffb98b42094ca1cc97da52114bbbc94fedb553899b1f18c26a349e186bba6e27922a89426bd61e8bc88b1f7832512dbe211b5f834e076e