addons/imguipanelmanager/imguipanelmanager.cpp: Updated a bit to reflect internal ImGui::Begin(...) changes in the upstream repository.
addons/imguipanelmanager/imguipanelmanager.cpp: Updated a bit to reflect internal ImGui::Begin(...) changes in the upstream repository.

Flix01/imgui,Flix01/imgui,Flix01/imgui