- Upgraded ImGui framework to 1.61 WIP, which adds a few improvements to context management.
- Removed from plugin implementation all the references to default ImGui context.
- Removed from ImGui Context Proxy destructor code switching to default ImGui context (there is no default context and decision is left for a higher level manager when to switch and to what context).
- Added own font atlas that is shared by all contexts and used explicitly to create Slate resources.
- Removed from ImGui Context Proxy workaround that was previously needed for copying cursor data to dynamically created contexts.
- Removed explicit saving of context settings before its destruction as this is now handled in ImGui::DestroyContext().
- Reimplemented ImGuiImplementation::GetCursorData() to use new interface.
