macOS: fix QFileOpenEvent routing for .qet/.elmt/.titleblock double-click

Root cause (see issue #218 discussion):
- QETApp::eventFiltrer (Q_OS_DARWIN) was correct in intent but never
  actually installed as an event filter anywhere, and its name didn't
  match the QObject::eventFilter virtual signature, so even if it had
  been installed it would never have been invoked as an override.
  Confirmed dead code.
- main.cpp instead routed Finder's QFileOpenEvent through
  MacOSXOpenEvent -> SingleApplication::sendMessage(), which is the
  secondary-to-primary IPC channel. Called from within the primary
  instance itself, sendMessage() fails silently and the file path is
  dropped, which is exactly what double-click does (Finder doesn't spawn
  a secondary process, the running instance receives the FileOpen event
  directly).
- openFiles() takes a QETArguments, not a QStringList. The dead code's
  openFiles(QStringList() << filename) only worked via an untested
  implicit QStringList -> QList<QString> -> QETArguments conversion
  chain.

Fix:
- Rename eventFiltrer -> eventFilter (qetapp.h/.cpp) so it's a proper
  override of QObject::eventFilter, and make it public so main.cpp can
  install it on QApplication.
- Build the QETArguments explicitly instead of relying on implicit
  conversion.
- main.cpp: drop MacOSXOpenEvent entirely (include + instantiation),
  install qetapp as the Q_OS_MACOS event filter on app right after
  QETApp qetapp; is constructed and before app.exec(). No race window:
  the event loop hasn't started, so no QFileOpenEvent can be delivered
  before the filter is installed.

QETArguments::handleFileArgument() already sorts files by extension
(.elmt, titleblock, else project) and openFiles() already fans out to
openProjectFiles()/openElementFiles()/openTitleBlockTemplateFiles()
accordingly, so this single fix covers .qet, .elmt and .titleblock
double-click/drag-to-dock on macOS, not just .qet.

SingleApplication's sendMessage/receivedMessage flow (used for CLI args
on all platforms) is untouched; this only touches the Q_OS_MACOS block,
so there is no behavior change on Windows/Linux.

Follow-up (not included here): the macOS Info.plist (misc/Info.plist)
has an empty CFBundleShortVersionString, which may affect macOS's
willingness to retain file associations across app updates.

sources/qetapp.h

@@ -178,10 +178,9 @@ class QETApp : public QObject
 		static QTextOrientationSpinBoxWidget *createTextOrientationSpinBoxWidget();
 		static TitleBlockTemplate *defaultTitleBlockTemplate();
 	
-	protected:
-		
 #ifdef Q_OS_DARWIN
-	bool eventFiltrer(QObject *object, QEvent *);
+	public:
+		bool eventFilter(QObject *object, QEvent *) override;
 #endif
 	
 		// attributes