Use dialog to select a zim file if no zim as been provide on command line.

It is better than crashing.

main.cpp

@@ -2,23 +2,34 @@
 #include "mainwindow.h"
 
 #include <QCommandLineParser>
+#include <QFileDialog>
+#include <iostream>
 
 int main(int argc, char *argv[])
 {
     KiwixApp a(argc, argv);
     KiwixApp::setApplicationName("kiwix-desktop");
 
-
     QCommandLineParser parser;
     parser.addPositionalArgument("zimfile", "The zim file");
 
     parser.process(a);
-    const QString zimfile = parser.positionalArguments().at(0);
-
-    a.openZimFile(zimfile);
+    QString zimfile;
+    auto positionalArguments = parser.positionalArguments();
+    if (positionalArguments.size() < 1){
+        zimfile = QFileDialog::getOpenFileName(nullptr,
+            "Open Zim",
+            QString(),
+            "ZimFile (*.zim*)");
+    } else {
+        zimfile = parser.positionalArguments().at(0);
+    }
+    std::string f = zimfile.toUtf8().constData();
+    std::cout << f << std::endl;
 
     MainWindow w;
     w.show();
+    a.openZimFile(zimfile);
 
     return a.exec();
 }