Merge #277: Do not use QClipboard::Selection on Windows and macOS.

7f3a598 qt: Do not use QClipboard::Selection on Windows and macOS. (Hennadii Stepanov)

Pull request description:

  Windows and macOS do [not support](https://doc.qt.io/qt-5/qclipboard.html#notes-for-windows-and-macos-users) the global mouse selection.

  Fixes #258.

ACKs for top commit:
  promag:
    Code review ACK 7f3a598.
  jarolrod:
    ACK 7f3a598

Tree-SHA512: be2beeef7d25af6f4d4a4548325d8d29f08e4342f499666bc4a670ed468a63195d514077c2cd0dba197e12bd43316fd3e2813cdc0954364b6aa4ae6b90c118bf

src/qt/guiutil.cpp

@@ -630,8 +630,11 @@ bool SetStartOnSystemStartup(bool fAutoStart) { return false; }
 
 void setClipboard(const QString& str)
 {
-    QApplication::clipboard()->setText(str, QClipboard::Clipboard);
-    QApplication::clipboard()->setText(str, QClipboard::Selection);
+    QClipboard* clipboard = QApplication::clipboard();
+    clipboard->setText(str, QClipboard::Clipboard);
+    if (clipboard->supportsSelection()) {
+        clipboard->setText(str, QClipboard::Selection);
+    }
 }
 
 fs::path qstringToBoostPath(const QString &path)