Fix build with Qt 5.11 (backported from upstream commits)

--- plugins/itemencrypted/itemencrypted.cpp.orig	2018-09-07 13:52:55 UTC
+++ plugins/itemencrypted/itemencrypted.cpp
@@ -30,9 +30,11 @@
 #include "gui/iconwidget.h"
 #include "item/serialize.h"
 
+#include <QAbstractItemModel>
 #include <QDir>
 #include <QFile>
 #include <QLabel>
+#include <QModelIndex>
 #include <QTextEdit>
 #include <QtPlugin>
 #include <QVBoxLayout>
--- src/gui/commanddialog.cpp.orig	2018-09-07 13:54:06 UTC
+++ src/gui/commanddialog.cpp
@@ -31,6 +31,7 @@
 #include "gui/icons.h"
 #include "platform/platformnativeinterface.h"
 
+#include <QAction>
 #include <QFileDialog>
 #include <QMenu>
 #include <QMessageBox>
--- src/gui/configtabappearance.cpp.orig	2018-09-07 14:05:13 UTC
+++ src/gui/configtabappearance.cpp
@@ -32,6 +32,7 @@
 #include "platform/platformnativeinterface.h"
 
 #include <QAbstractScrollArea>
+#include <QAction>
 #include <QColorDialog>
 #include <QFileDialog>
 #include <QFontDialog>
--- src/gui/processmanagerdialog.cpp.orig	2018-09-07 13:56:41 UTC
+++ src/gui/processmanagerdialog.cpp
@@ -26,6 +26,7 @@
 #include "gui/icons.h"
 #include "gui/windowgeometryguard.h"
 
+#include <QAction>
 #include <QDateTime>
 #include <QPushButton>
 
--- src/gui/traymenu.cpp.orig	2018-09-07 13:55:30 UTC
+++ src/gui/traymenu.cpp
@@ -25,6 +25,7 @@
 #include "platform/platformnativeinterface.h"
 #include "platform/platformwindow.h"
 
+#include <QAction>
 #include <QApplication>
 #include <QKeyEvent>
 #include <QModelIndex>
--- src/gui/traymenu.h.orig	2018-09-07 13:57:46 UTC
+++ src/gui/traymenu.h
@@ -23,6 +23,7 @@
 #include <QMenu>
 #include <QPointer>
 
+class QAction;
 class QModelIndex;
 
 class TrayMenu : public QMenu