UPD & FIX: handle Splash Form consistently across all platforms, especially on MacOS

1. more Lazarus’ idioms for SplashForm (like Lazarus IDE)
2. SplashForm can be shown correctly on MacOS
3. improved display of SplashForm in Gt2k
4. tested on Win32, Gtk2, Qt5, Cocoa

(cherry picked from commit 623185b685)

src/doublecmd.lpr

@@ -230,13 +230,6 @@ begin
       // in Application.CreateForm above.
       uKeyboard.HookKeyboardLayoutChanged;
 
-      if (gSplashForm) and (not CommandLineParams.NoSplash) then
-      begin
-        // We may now remove the starting splash screen, most of the application has been started now
-        frmStartingSplash.Close;
-        frmStartingSplash.Release;
-      end;
-
       frmMain.ShowOnTop;
       {$IFDEF LCLCOCOA}
       frmMain.RestoreWindow;

src/fstartingsplash.lfm

@@ -1019,6 +1019,8 @@ object frmStartingSplash: TfrmStartingSplash
   OnCreate = FormCreate
   Position = poScreenCenter
   LCLVersion = '2.0.12.0'
+  OnCreate = FormCreate
+  OnHide = FormHide
   object pnlInfo: TPanel
     AnchorSideLeft.Control = Owner
     AnchorSideTop.Control = Owner

src/fstartingsplash.pas

@@ -27,6 +27,8 @@ type
     pnlVersionInfos: TPanel;
     pnlInfo: TPanel;
     procedure FormCreate(Sender: TObject);
+    procedure FormClose(Sender: TObject; var CloseAction: TCloseAction);
+    procedure FormHide(Sender: TObject);
   private
     { private declarations }
   public
@@ -58,5 +60,16 @@ begin
   lblWidgetsetVer.Caption    := WSVersion;
 end;
 
+procedure TfrmStartingSplash.FormClose(Sender: TObject;
+  var CloseAction: TCloseAction);
+begin
+  CloseAction:= caFree;
+end;
+
+procedure TfrmStartingSplash.FormHide(Sender: TObject);
+begin
+  close();
+end;
+
 end.