ui: fixed crash on Wayland resizing the pop-ups

- Fixed crash on Wayland (Fedora 37) when setting the minimum and
   maximum size of a pop-up. setMinimumSize() makes crash the GUI.
 - With the above fix, we don't need to set QT_QPA_PLATFORM
   (Closes: #794)

ui/bin/opensnitch-ui

@@ -69,7 +69,6 @@ Examples:
         import faulthandler
         faulthandler.enable()
 
-    os.environ["QT_QPA_PLATFORM"] = "xcb"
     os.environ["QT_AUTO_SCREEN_SCALE_FACTOR"] = "1"
     if supported_qt_version(5,6,0):
         try:

ui/opensnitch/dialogs/prompt.py

@@ -152,8 +152,7 @@ class PromptDialog(QtWidgets.QDialog, uic.loadUiType(DIALOG_UI_PATH)[0]):
             self._width = self.width()
             self._height = self.height()
 
-        self.setMinimumSize(self._width, self._height)
-        self.setMaximumSize(self._width, self._height)
+        self.resize(QtCore.QSize(self._width, self._height))
 
     def move_popup(self):
         popup_pos = self._cfg.getInt(self._cfg.DEFAULT_POPUP_POSITION)