message: Add "Drag folders or files here" to source lists
diff: #include "propertiesdlg.h"
  #include "colorscheme.h"
  +#include "customcontrols.h"
  #include "hidpi.h"
  #include "language.h"
  #include "str_helpers.h"
  @@ -304,19 +305,28 @@ public:
  Remove(s);
  });
  -#if 0
  - // TODO: Add background overlay with instructions:
  - _("<big>Drag and Drop Folders Here</big>\n\nor use the + button")
  - _("<big>Drag and drop folders here</big>\n\nor use the + button")
  + m_placeholder = new wxStaticText(this, wxID_ANY, MSW_OR_OTHER(_("Drag folders or files here"), _("Drag Folders or Files Here")));
  + m_placeholder->SetForegroundColour(ExplanationLabel::GetTextColor());
  +#ifdef __WXOSX__
  + m_placeholder->SetWindowVariant(wxWINDOW_VARIANT_NORMAL);
  #endif
  + m_list->Bind(wxEVT_SIZE, &PathsList::OnListResized, this);
  m_list->Bind(wxEVT_CONTEXT_MENU, &PathsList::OnRightClick, this);
  +
  +#ifdef __WXMSW__
  + m_list->Bind(wxEVT_SET_FOCUS, [=](wxFocusEvent& e){
  + e.Skip();
  + m_placeholder->Lower(); // move to the top in Z-order, above the list (yeah, really)
  + });
  +#endif
  }
  void UpdateFromData()
  {
  + auto& array = Array();
  m_list->Clear();
  - for (auto& p: Array())
  + for (auto& p: array)
  {
  wxString s;
  if (wxIsWild(p))
  @@ -325,6 +335,8 @@ public:
  s = RelativePath(p, m_data->basepath, wxPATH_NATIVE);
  m_list->Append(bidi::platform_mark_direction(s));
  }
  +
  + m_placeholder->Show(array.empty());
  }
  void Add(const wxArrayString& files)
  @@ -378,6 +390,14 @@ protected:
  PathsList *m_parent;
  };
  + void OnListResized(wxSizeEvent& e)
  + {
  + e.Skip();
  + wxSize size = m_list->GetSize();
  + size -= m_placeholder->GetSize();
  + m_placeholder->SetPosition(m_list->GetPosition() + size / 2);
  + }
  +
  void OnAddMenu(wxCommandEvent& e)
  {
  static const auto idFolder = wxNewId();
  @@ -488,6 +508,7 @@ protected:
  std::shared_ptr<PathsData> m_data;
  wxListBox *m_list;
  + wxStaticText *m_placeholder;
  };
  class PropertiesDialog::SourcePathsList : public PropertiesDialog::PathsList
  