Qt5: fix deprecation warnings for Qt 5.15

+ replace QDirModel with QFileSystemModel
+ QProcess::start(QString, OpenMode) is deprecated
+ QByteArray::append is deprecated
+ QPixmap* QLabel::pixmap() is deprecated

src/Gui/TaskElementColors.cpp

@@ -263,7 +263,7 @@ public:
                    boost::starts_with(msg.pSubName,editSub))
                 {
                     for(auto item : ui->elementList->findItems(
-                                QString::fromLatin1(msg.pSubName-editSub.size()),0))
+                                QString::fromLatin1(msg.pSubName-editSub.size()), Qt::MatchExactly))
                         item->setSelected(msg.Type==SelectionChanges::AddSelection);
                 }
             }