Library: add a shortcut to focus search line

The Ctrl+F shortcut gives focus to a search bar in many applications.
In this case it allows to search the library without touching a mouse.

YACReaderMacOSXSearchLineEdit::setFocus() will have to be implemented to
make the shortcut work on macOS.

custom_widgets/yacreader_macosx_toolbar.mm

@@ -316,6 +316,12 @@ YACReaderMacOSXSearchLineEdit::YACReaderMacOSXSearchLineEdit()
     nstextfield = searchEdit;
 }
 
+void YACReaderMacOSXSearchLineEdit::setFocus(Qt::FocusReason reason)
+{
+    Q_UNUSED(reason)
+    // TODO: implement
+}
+
 void *YACReaderMacOSXSearchLineEdit::getNSTextField()
 {
     return nstextfield;