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/gui/Application.cpp

@@ -49,7 +49,6 @@ namespace
 
 Application::Application(int& argc, char** argv)
     : QApplication(argc, argv)
-    , m_mainWindow(nullptr)
 #ifdef Q_OS_UNIX
     , m_unixSignalNotifier(nullptr)
 #endif
@@ -143,16 +142,6 @@ Application::~Application()
     }
 }
 
-QWidget* Application::mainWindow() const
-{
-    return m_mainWindow;
-}
-
-void Application::setMainWindow(QWidget* mainWindow)
-{
-    m_mainWindow = mainWindow;
-}
-
 bool Application::event(QEvent* event)
 {
     // Handle Apple QFileOpenEvent from finder (double click on .kdbx file)