Opening a file when another instance is already is running sometimes fails

Opening a file when another instance is already is running sometimes fail, ending up with just raising a window and not actually opening the file (ie. `MainWindow::bringWindowToForeground()` runs in primary instance, but it's not followed by `NotepadNextApplication::openFiles(const QStringList&)` or anything else )

It might be my imagination, but the more tabs are open the higher the chances of it failing

NotepadNext 0.7 running on linux

New file opened using `NotepadNext "/home/user/dir/foo bar.txt"`