isModuleInstalled added to BackendModel
Changed name to isInstalledModule()

Conform functions like BackendAuthentication::isAllowedModule() and
isAllowedAction().
Removed BackendExtensionsModel::isModuleInstalled()
Replaced BackendExtensionsModel::isModuleInstalled() by BackendModel::isInstalledModule()
renamed isInstalledModule to isModuleInstalled
Resolve conflicts.