ui: fixed Linux issues

* no outline style for push buttons
* show error when not using Sourcetrail.sh script in console instead of message box, which won't load

src/lib_gui/qt/window/QtMainWindow.cpp

@@ -5,7 +5,6 @@
 #include <QDockWidget>
 #include <QDir>
 #include <QMenuBar>
-#include <QMessageBox>
 #include <QSettings>
 #include <QTimer>
 
@@ -112,13 +111,6 @@ QtMainWindow::QtMainWindow()
 	setWindowIcon(QIcon((ResourcePaths::getGuiPath().str() + "icon/logo_1024_1024.png").c_str()));
 	setWindowFlags(Qt::Widget);
 
-#ifdef __linux__
-	if (std::getenv("SOURCETRAIL_VIA_SCRIPT") == nullptr)
-	{
-	   QMessageBox::warning(this, "Run Sourcetrail via Script", "Please run Sourcetrail via Sourcetrail.sh");
-	}
-#endif
-
 	QApplication* app = dynamic_cast<QApplication*>(QCoreApplication::instance());
 	app->installEventFilter(new MouseReleaseFilter(this));