Allow calling EnableSystemTheme(false) before creating the window

This is important as enabling the system theme results in changes to the
native ListView control appearance that are not undone later even if the
system theme is disabled. Notably, the item rectangle width is reduced
by 2 pixels when system theme is enabled and it's not increased to its
original value even when it's disabled later, resulting in gaps between
items through which the control background shows, for example. This also
makes items drawn using our own HandleItemPaint() slightly, but
noticeably, larger than the items using standard appearance, which looks
bad.

All these problems can be avoided if we skip enabling the system theme
in the first place if EnableSystemTheme(false) had been called before
creating the control, so support doing it like this and document that
this is the preferred way of disabling the system theme use.

Closes #17404, #18296.

include/wx/systhemectrl.h

@@ -21,7 +21,21 @@ class WXDLLIMPEXP_FWD_CORE wxWindow;
 class WXDLLIMPEXP_CORE wxSystemThemedControlBase
 {
 public:
-    wxSystemThemedControlBase() { }
+    wxSystemThemedControlBase()
+    {
+#ifdef wxHAS_SYSTEM_THEMED_CONTROL
+        m_systemThemeDisabled = false;
+#endif // wxHAS_SYSTEM_THEMED_CONTROL
+    }
+
+    bool IsSystemThemeDisabled() const
+    {
+#ifdef wxHAS_SYSTEM_THEMED_CONTROL
+        return m_systemThemeDisabled;
+#else // !wxHAS_SYSTEM_THEMED_CONTROL
+        return false;
+#endif // wxHAS_SYSTEM_THEMED_CONTROL/!wxHAS_SYSTEM_THEMED_CONTROL
+    }
 
     virtual ~wxSystemThemedControlBase() { }
 
@@ -36,6 +50,11 @@ protected:
     (bool WXUNUSED(enable), wxWindow* WXUNUSED(window)) { }
 #endif // wxHAS_SYSTEM_THEMED_CONTROL
 
+private:
+#ifdef wxHAS_SYSTEM_THEMED_CONTROL
+    bool m_systemThemeDisabled;
+#endif // wxHAS_SYSTEM_THEMED_CONTROL
+
     wxDECLARE_NO_COPY_CLASS(wxSystemThemedControlBase);
 };
 
@@ -54,6 +73,14 @@ public:
     }
 
 protected:
+    void EnableSystemThemeByDefault()
+    {
+        // Check if the system theme hadn't been explicitly disabled before
+        // enabling it by default.
+        if ( !this->IsSystemThemeDisabled() )
+            DoEnableSystemTheme(true, this);
+    }
+
     wxDECLARE_NO_COPY_TEMPLATE_CLASS(wxSystemThemedControl, C);
 };