launcher : move call to ShowSplashScreen and BackgroundInitialization to PreStart()
It doesn't change behaviour of launcher in beremiz, but may have some impact on some customizations.
Some customizations need to squeeze some bitmap folder init in between app creation and ShowSplashScreen

                  <xsd:element name="Linux">
                    <xsd:complexType>
                      %(toolchain_gcc)s
                    </xsd:complexType>
                  </xsd:element>