BeginMenu(): fixed logic to distinguish vertical menu from one layed out in a menu bar. Makes MenuItem() in a regular window behave more consistently, and this will be also needed by upcoming menu-navigation changes in the nav branch. (#126, #787)

imgui.cpp

@@ -10029,7 +10029,7 @@ bool ImGui::BeginMenu(const char* label, bool enabled)
         g.NavWindow = backed_nav_window;
 
     bool want_open = false, want_close = false;
-    if (window->Flags & (ImGuiWindowFlags_Popup|ImGuiWindowFlags_ChildMenu))
+    if (window->DC.LayoutType != ImGuiLayoutType_Horizontal) // (window->Flags & (ImGuiWindowFlags_Popup|ImGuiWindowFlags_ChildMenu))
     {
         // Implement http://bjk5.com/post/44698559168/breaking-down-amazons-mega-dropdown to avoid using timers, so menus feels more reactive.
         bool moving_within_opened_triangle = false;