Reader: make 3 keyboard shortcuts work with non-Latin layouts

MainWindowViewer::keyPressEvent()'s custom matching of these shortcuts
does not leverage all the features of standard Qt shortcut matching. As
a result, the corresponding actions cannot be triggered when their
assigned shortcuts consist of a single Latin letter without modifiers
and e.g. Ukrainian keyboard layout is active.

Furthermore, some key presses (e.g. Scroll Lock in my customized
keyboard layout) set QKeyEvent::key() to 0, which the custom matching
considers equal to an unassigned shortcut. So an action without shortcut
is triggered by such a key press.

Adding these 3 actions to MainWindowViewer and connecting the
corresponding slots to their triggered() signals allows to remove the
custom matching code and thus eliminates both of its issues.

YACReader/main_window_viewer.h

@@ -4,7 +4,6 @@
 #include <QScrollArea>
 #include <QToolBar>
 #include <QAction>
-#include <QKeyEvent>
 #include <QMouseEvent>
 #include <QCloseEvent>
 #include <QSettings>
@@ -165,7 +164,6 @@ private:
     void getSiblingComics(QString path, QString currentComic);
 
     //! Manejadores de evento:
-    void keyPressEvent(QKeyEvent *event) override;
     // void resizeEvent(QResizeEvent * event);
     void mouseDoubleClickEvent(QMouseEvent *event) override;
     void dropEvent(QDropEvent *event) override;