partial Merge bitcoin-core/gui#30: Disable the main window toolbar wh…

…en the modal overlay is shown

BACKPORT COMMENT
Central widget is null at this point in our implementation. Also, we have no issues with the toolbar which original PR was aimed to fix.
Code is not presented in this commit is DNM, merged only changes in qt/modaloverlay (except b4c1af9#diff-bd2b12135c597b047fffc3b7a8e86e4a965c86ec7763f6ea885b6d8f6f702d10R40-R42 which is unused)

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d0cc1f6 qt: Disable toolbar when overlay is shown (Hennadii Stepanov)
e74cd20 qt, refactor: Cleanup ModalOverlay slots (Hennadii Stepanov)

Pull request description:

  Keeping the main window toolbar activated while the modal overlay is shown could create the appearance of the non-responsive GUI.

  Fixes dashpay#22.

  ---

  On master (ca05588):

  ![Screenshot from 2020-07-11 13-07-00](https://user-images.githubusercontent.com/32963518/87221791-7504e100-c377-11ea-9689-ddd4b21b98f9.png)

  With this PR:

  ![Screenshot from 2020-07-11 13-07-39](https://user-images.githubusercontent.com/32963518/87221803-8817b100-c377-11ea-92c8-3602dc4d2451.png)

ACKs for top commit:
  harding:
    Tested ACK d0cc1f6.  Tested on Linux/X11 as much as I could given it's a pretty small change; seems like a nice improvement.  I'm not experienced in Qt, but I don't see anything obviously problematic about the code.
  jonatack:
    ACK d0cc1f6 tested on Debian 5.7.6-1 (2020-06-24) x86_64 GNU/Linux
  LarryRuane:
    ACK d0cc1f6 tested on Ubuntu 18.04.4 LTS

Tree-SHA512: e371b34231c01e77118deb100e0f280ba1cdef54e317f7f7d6ac322598bda811bd1bfe3035e90d87f8267f4f5d2095d34a8136911159db63694fd1b1b11335a1

src/qt/modaloverlay.h

@@ -25,17 +25,18 @@ class ModalOverlay : public QWidget
     explicit ModalOverlay(bool enable_wallet, QWidget *parent);
     ~ModalOverlay();
 
-public Q_SLOTS:
     void tipUpdate(int count, const QDateTime& blockDate, double nVerificationProgress);
     void setKnownBestHeight(int count, const QDateTime& blockDate);
 
-    void toggleVisibility();
     // will show or hide the modal layer
     void showHide(bool hide = false, bool userRequested = false);
-    void closeClicked();
     void hideForever();
     bool isLayerVisible() const { return layerIsVisible; }
 
+public Q_SLOTS:
+    void toggleVisibility();
+    void closeClicked();
+
 protected:
     bool eventFilter(QObject * obj, QEvent * ev) override;
     bool event(QEvent* ev) override;