Add "Add Models" to plate and background context menu

[yw4z]

Adds "Add Model" item for plate and background context menus

• One of the most used functions like arrange, split or etc. they are already exist on toolbar and context menu
• Knowing add / import function on context menu is useful and adds a alternative way to import model
• you don't have to move mouse to toolbar. reduces mouse movements
• I'm sure many people will found useful. especially people that came from cura. I'm still searching this item on context menu
• I added menu item to end of menu to keep same order otherwise old users will click to "add model" every time instead "add primitive"
• "Add models" already exist in translations. Does not require additional translations

Code can be shorter if it was used like this

#ifdef __WINDOWS__
    auto add_icon = "menu_add_part"
#else
    auto add_icon = ""
#endif
append_submenu(menu, sub_menu_primitives, wxID_ANY, _L("Add Primitive"), "", add_icon,
    []() {return true; }, m_parent);
append_submenu(menu, sub_menu_handy, wxID_ANY, _L("Add Handy models"), "", add_icon,
    []() {return true; }, m_parent);
// ORCA: Add Model
append_menu_item(menu, wxID_ANY, _L("Add Model"), "",
    [](wxCommandEvent&) { plater()->add_file(); }, add_icon, menu,
    []() {  return wxGetApp().plater()->can_add_model(); }, m_parent);

[SoftFever]

Good one.
Thanks!