Rename pages to maintain the coherence with the documentation.

 InstallationModePage.nsh -> InstallationTypePage.nsh
 InstallationModePageLangStrings.nsh -> InstallationTypePageLangStrings.nsh
 InstallationModePage.nsddef -> InstallationTypePage.nsddef
 InstallationModePage.nsdinc -> InstallationTypePage.nsdinc