refactor: add explicit this-> where possible

I have knowingly skipped some files/portions of files where this would
create merge conflicts for other open PRs.

src/widgets/settingspages/ExternalToolsPage.cpp

@@ -90,7 +90,7 @@ ExternalToolsPage::ExternalToolsPage()
     y->addWidget(z);
     this->setLayout(y);
 
-    this->initLayout(*view);
+    this->initLayout(*this->view);
 }
 
 bool ExternalToolsPage::filterElements(const QString &query)

src/widgets/settingspages/KeyboardSettingsPage.cpp

@@ -77,7 +77,7 @@ KeyboardSettingsPage::KeyboardSettingsPage()
         }
     });
 
-    QObject::connect(view_->getTableView(), &QTableView::doubleClicked,
+    QObject::connect(this->view_->getTableView(), &QTableView::doubleClicked,
                      [view, model](const QModelIndex &clicked) {
                          tableCellClicked(clicked, view, model);
                      });

src/widgets/settingspages/SettingWidget.cpp

@@ -35,7 +35,7 @@ SettingWidget::SettingWidget(const QString &mainKeyword)
     this->vLayout->setContentsMargins(0, 0, 0, 0);
 
     this->hLayout->setContentsMargins(0, 0, 0, 0);
-    this->vLayout->addLayout(hLayout);
+    this->vLayout->addLayout(this->hLayout);
 
     this->keywords.append(mainKeyword);
 }