[webgui] let force longpoll engine for RWebWindow connection

In normal situation websockets tried first and only if fails,
fallback to longpoll. WebGui.WSLongpoll: yes in rootrc file
force longpoll from the beginning.
Handle all RWebWindow parameters in one place.
Provide WSKind and WSPlatform methods for THttpCallArg
Harmonize boolean flags handling in RWebWindowsManager