Add support for building with mingw-w64 GCC. Fix warnings

Pdf4QtViewer/Pdf4QtViewer.pro

@@ -20,7 +20,8 @@ QT += core gui widgets winextras printsupport texttospeech network xml
 TARGET = Pdf4QtViewer
 TEMPLATE = lib
 
-win32:TARGET_EXT = .dll
+include(../Pdf4Qt.pri)
+
 VERSION = 1.1.0
 
 DEFINES += PDF4QTVIEWER_LIBRARY
@@ -30,7 +31,9 @@ QMAKE_TARGET_COPYRIGHT = "(c) Jakub Melka 2018-2021"
 
 DEFINES += QT_DEPRECATED_WARNINGS
 
-QMAKE_CXXFLAGS += /std:c++latest /utf-8
+win32-*g++|unix: {
+    LIBS += -ltbb
+}
 
 INCLUDEPATH += $$PWD/../PDF4QtLib/Sources
 DESTDIR = $$OUT_PWD/..

Pdf4QtViewer/pdfdocumentpropertiesdialog.cpp

@@ -361,7 +361,7 @@ void PDFDocumentPropertiesDialog::initializeFonts(const pdf::PDFDocument* docume
                                     }
                                 }
                             }
-                            catch (pdf::PDFException)
+                            catch (const pdf::PDFException &)
                             {
                                 // Do nothing, some error occured, continue with next font
                                 continue;
@@ -370,7 +370,7 @@ void PDFDocumentPropertiesDialog::initializeFonts(const pdf::PDFDocument* docume
                     }
                 }
             }
-            catch (pdf::PDFException)
+            catch (const pdf::PDFException &)
             {
                 // Do nothing, some error occured
             }

Pdf4QtViewer/pdfprogramcontroller.cpp

@@ -318,8 +318,8 @@ PDFProgramController::PDFProgramController(QObject* parent) :
     m_undoRedoManager(nullptr),
     m_recentFileManager(new PDFRecentFileManager(this)),
     m_optionalContentActivity(nullptr),
-    m_futureWatcher(nullptr),
     m_textToSpeech(nullptr),
+    m_futureWatcher(nullptr),
     m_CMSManager(new pdf::PDFCMSManager(this)),
     m_toolManager(nullptr),
     m_annotationManager(nullptr),

Pdf4QtViewer/pdfsidebarwidget.cpp

@@ -663,7 +663,7 @@ void PDFSidebarWidget::onAttachmentCustomContextMenuRequested(const QPoint& pos)
                             QMessageBox::critical(this, tr("Error"), tr("Failed to save attachment to file. %1").arg(file.errorString()));
                         }
                     }
-                    catch (pdf::PDFException e)
+                    catch (const pdf::PDFException &e)
                     {
                         QMessageBox::critical(this, tr("Error"), tr("Failed to save attachment to file. %1").arg(e.getMessage()));
                     }

Pdf4QtViewer/pdfviewersettings.cpp

@@ -263,11 +263,11 @@ PDFViewerSettings::Settings::Settings() :
     m_allowLaunchApplications(true),
     m_allowLaunchURI(true),
     m_allowDeveloperMode(false),
+    m_multithreadingStrategy(pdf::PDFExecutionPolicy::Strategy::AlwaysMultithreaded),
     m_compiledPageCacheLimit(128 * 1024),
     m_thumbnailsCacheLimit(PIXMAP_CACHE_LIMIT),
     m_fontCacheLimit(pdf::DEFAULT_FONT_CACHE_LIMIT),
     m_instancedFontCacheLimit(pdf::DEFAULT_REALIZED_FONT_CACHE_LIMIT),
-    m_multithreadingStrategy(pdf::PDFExecutionPolicy::Strategy::AlwaysMultithreaded),
     m_speechRate(0.0),
     m_speechPitch(0.0),
     m_speechVolume(1.0),

Pdf4QtViewer/pdfviewersettingsdialog.cpp

@@ -69,9 +69,9 @@ PDFViewerSettingsDialog::PDFViewerSettingsDialog(const PDFViewerSettings::Settin
     m_settings(settings),
     m_cmsSettings(cmsSettings),
     m_otherSettings(otherSettings),
-    m_certificateStore(certificateStore),
     m_actions(),
     m_isLoadingData(false),
+    m_certificateStore(certificateStore),
     m_enabledPlugins(enabledPlugins),
     m_plugins(plugins),
     m_networkAccessManager(nullptr),