Merge #17696: qt: Force set nPruneSize in QSettings after the intro dialog

af112ab628 qt: Rename SetPrune() to InitializePruneSetting() (Hennadii Stepanov)
b0bfbe5028 refactor: Drop `bool force' parameter (Hennadii Stepanov)
68c9bbe9bc qt: Force set nPruneSize in QSettings after intro (Hennadii Stepanov)
a82bd8fa57 util: Replace magics with DEFAULT_PRUNE_TARGET_GB (Hennadii Stepanov)

Pull request description:

  On master (5622d8f315), having `QSettings` set already
  ```
  $ grep nPruneSize ~/.config/Bitcoin/Bitcoin-Qt-testnet.conf
  nPruneSize=6
  ```

  enabling prune option in the intro dialog

  ```
  $ ./src/qt/bitcoin-qt -choosedatadir -testnet
  ```
  ![DeepinScreenshot_select-area_20191208120425](https://user-images.githubusercontent.com/32963518/70388183-eed68580-19b6-11ea-9aa1-f9ad9aaa68a6.png)

  has no effect:
  ```
  $ grep Prune ~/.bitcoin/testnet3/debug.log
  2019-12-08T10:04:41Z Prune configured to target 5722 MiB on disk for block and undo files.
  ```

  ---

  With this PR:
  ```
  $ grep Prune ~/.bitcoin/testnet3/debug.log
  2019-12-08T10:20:35Z Prune configured to target 1907 MiB on disk for block and undo files.
  ```

  This PR has been split of #17453 (the first two commits) as it fixes an orthogonal bug.

  Refs:
  - https://github.com/bitcoin/bitcoin/pull/17453#discussion_r345424240
  - https://github.com/bitcoin/bitcoin/pull/17453#discussion_r350960201

ACKs for top commit:
  Sjors:
    Code review re-ACK af112ab628
  ryanofsky:
    Code review ACK af112ab628. Just suggested changes since last review (thanks!)
  promag:
    Tested ACK af112ab628. Latest suggestions and changes look good to me.

Tree-SHA512: 8ddad34b30dcc2cdcad6678ba8a0b36fa176e4e3465862ef6eee9be0f98d8146705138c9c7995dd8c0990af41078ca743fef1a90ed9240081f052f32ddec72b9

src/qt/optionsmodel.h

@@ -73,7 +73,8 @@ public:
     const QString& getOverriddenByCommandLine() { return strOverriddenByCommandLine; }
 
     /* Explicit setters */
-    void SetPrune(bool prune, bool force = false);
+    void SetPruneEnabled(bool prune, bool force = false);
+    void SetPruneTargetGB(int prune_target_gb, bool force = false);
 
     /* Restart flag helper */
     void setRestartRequired(bool fRequired);