Use QApplication::screens().first() instead of deprecated methods

Also follows-up moving away from QDesktop widget from c99f12e8fe
Note: `QScreen::availableGeometry()` doesn't take the parent argument as opposed to `QDesktopWidget::availableGeometry(QWidget &xd)` - no idea if not passing an argument breaks anything, but just for the sake of having Chatterino compile I didn't mind it that much while fixing compilation.

Reference:
- https://doc.qt.io/qt-6/widgets-changes-qt6.html#qdesktopwidget-and-qapplication-desktop

src/widgets/BaseWindow.cpp

@@ -15,6 +15,7 @@
 #include <QDebug>
 #include <QFont>
 #include <QIcon>
+#include <QScreen>
 #include <functional>
 
 #ifdef CHATTERINO
@@ -601,10 +602,10 @@ void BaseWindow::moveIntoDesktopRect(QWidget *parent, QPoint point)
         return;
 
     // move the widget into the screen geometry if it's not already in there
-    QDesktopWidget *desktop = QApplication::desktop();
+    auto *desktop = QApplication::screens().first();
     QPoint globalCursorPos = QCursor::pos();
 
-    QRect s = desktop->availableGeometry(parent);
+    QRect s = desktop->availableGeometry(/*parent*/);
 
     bool stickRight = false;
     bool stickBottom = false;

src/widgets/dialogs/NotificationPopup.cpp

@@ -5,9 +5,7 @@
 #include "widgets/helper/ChannelView.hpp"
 
 #include <QApplication>
-#include <QDesktopWidget>
 #include <QScreen>
-
 #include <QVBoxLayout>
 
 namespace chatterino {
@@ -32,7 +30,7 @@ void NotificationPopup::updatePosition()
 {
     Location location = BottomRight;
 
-    QDesktopWidget *desktop = QApplication::desktop();
+    auto *desktop = QApplication::screens().first();
     const QRect rect = desktop->availableGeometry();
 
     switch (location)

src/widgets/helper/Button.cpp

@@ -2,7 +2,6 @@
 
 #include <QApplication>
 #include <QDebug>
-#include <QDesktopWidget>
 #include <QPainter>
 
 #include "BaseTheme.hpp"
@@ -345,7 +344,8 @@ void Button::showMenu()
         return;
 
     auto point = [this] {
-        auto bounds = QApplication::desktop()->availableGeometry(this);
+        auto bounds =
+            QApplication::screens().first()->availableGeometry(/*this*/);
 
         auto point = this->mapToGlobal(
             QPoint(this->width() - this->menu_->width(), this->height()));

src/widgets/splits/SplitHeader.cpp

@@ -22,7 +22,6 @@
 #include "widgets/splits/Split.hpp"
 #include "widgets/splits/SplitContainer.hpp"
 
-#include <QDesktopWidget>
 #include <QDrag>
 #include <QHBoxLayout>
 #include <QInputDialog>