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

bin/app/data/gui/main/main.css

@@ -4,6 +4,10 @@ QToolTip {
 	font-size: <setting:font_size>px;
 }
 
+QPushButton {
+	outline: none;
+}
+
 QMainWindow::separator {
 	background: <color:window/separator>;
 	width: 1px;