Fix size of Unsaved changes dialog.

Since Qt 6 produces warnings "QWindowsWindow::setGeometry: Unable to set geometry".

src/app/puzzle/vpmainwindow.cpp

@@ -1794,6 +1794,7 @@ auto VPMainWindow::MaybeSave() -> bool
         }
 
         messageBox->setWindowModality(Qt::ApplicationModal);
+        messageBox->setFixedSize(300, 85);
         const auto ret = static_cast<QMessageBox::StandardButton>(messageBox->exec());
 
         switch (ret)

src/app/tape/tkmmainwindow.cpp

@@ -1942,6 +1942,7 @@ auto TKMMainWindow::MaybeSave() -> bool
     }
 
     messageBox->setWindowModality(Qt::ApplicationModal);
+    messageBox->setFixedSize(300, 85);
     const auto ret = static_cast<QMessageBox::StandardButton>(messageBox->exec());
 
     switch (ret)

src/app/tape/tmainwindow.cpp

@@ -3474,6 +3474,7 @@ auto TMainWindow::MaybeSave() -> bool
     }
 
     messageBox->setWindowModality(Qt::ApplicationModal);
+    messageBox->setFixedSize(300, 85);
     const auto ret = static_cast<QMessageBox::StandardButton>(messageBox->exec());
 
     switch (ret)

src/app/valentina/mainwindow.cpp

@@ -5756,6 +5756,7 @@ auto MainWindow::MaybeSave() -> bool
         }
 
         messageBox->setWindowModality(Qt::ApplicationModal);
+        messageBox->setFixedSize(300, 85);
         const auto ret = static_cast<QMessageBox::StandardButton>(messageBox->exec());
 
         switch (ret)

src/libs/vlayout/dialogs/watermarkwindow.cpp

@@ -502,6 +502,7 @@ auto WatermarkWindow::MaybeSave() -> bool
         }
 
         messageBox->setWindowModality(Qt::ApplicationModal);
+        messageBox->setFixedSize(300, 85);
         const auto ret = static_cast<QMessageBox::StandardButton>(messageBox->exec());
 
         switch (ret)