Add search help pop-up

* Support ! modifier (same as '-')
* Create reusable PopupHelpWidget as self-contained popup that can
be positioned around a parent widget and will follow the movement
and sizing of the window
* Eliminated KEEPASSXC_MAIN_WINDOW macro and replaced with
getMainWindow() function
* Add tests to cover search help show/hide

src/main.cpp

@@ -111,7 +111,6 @@ int main(int argc, char** argv)
     }
 
     MainWindow mainWindow;
-    app.setMainWindow(&mainWindow);
     QObject::connect(&app, SIGNAL(anotherInstanceStarted()), &mainWindow, SLOT(bringToFront()));
     QObject::connect(&app, SIGNAL(applicationActivated()), &mainWindow, SLOT(bringToFront()));
     QObject::connect(&app, SIGNAL(openFile(QString)), &mainWindow, SLOT(openDatabase(QString)));