Added Control.preferredSize() and preferredSize() for all the standard Controls.

button.go

@@ -67,3 +67,11 @@ func (b *Button) setRect(x int, y int, width int, height int) error {
 
 	return b.sysData.setRect(x, y, width, height)
 }
+
+func (b *Button) preferredSize() (width int, height int, err error) {
+	b.lock.Lock()
+	defer b.lock.Unlock()
+
+	width, height = b.sysData.preferredSize()
+	return
+}

checkbox.go

@@ -74,3 +74,11 @@ func (c *Checkbox) setRect(x int, y int, width int, height int) error {
 
 	return c.sysData.setRect(x, y, width, height)
 }
+
+func (c *Checkbox) preferredSize() (width int, height int, err error) {
+	c.lock.Lock()
+	defer c.lock.Unlock()
+
+	width, height = c.sysData.preferredSize()
+	return
+}

combobox.go

@@ -110,3 +110,11 @@ func (c *Combobox) setRect(x int, y int, width int, height int) error {
 
 	return c.sysData.setRect(x, y, width, height)
 }
+
+func (c *Combobox) preferredSize() (width int, height int, err error) {
+	c.lock.Lock()
+	defer c.lock.Unlock()
+
+	width, height = c.sysData.preferredSize()
+	return
+}

control.go

@@ -9,4 +9,5 @@ import (
 type Control interface {
 	make(window *sysData) error
 	setRect(x int, y int, width int, height int) error
+	preferredSize() (width int, height int, err error)
 }

label.go

@@ -62,3 +62,11 @@ func (l *Label) setRect(x int, y int, width int, height int) error {
 
 	return l.sysData.setRect(x, y, width, height)
 }
+
+func (l *Label) preferredSize() (width int, height int, err error) {
+	l.lock.Lock()
+	defer l.lock.Unlock()
+
+	width, height = l.sysData.preferredSize()
+	return
+}

lineedit.go

@@ -64,3 +64,11 @@ func (l *LineEdit) setRect(x int, y int, width int, height int) error {
 
 	return l.sysData.setRect(x, y, width, height)
 }
+
+func (l *LineEdit) preferredSize() (width int, height int, err error) {
+	l.lock.Lock()
+	defer l.lock.Unlock()
+
+	width, height = l.sysData.preferredSize()
+	return
+}

listbox.go

@@ -110,3 +110,11 @@ func (l *Listbox) setRect(x int, y int, width int, height int) error {
 
 	return l.sysData.setRect(x, y, width, height)
 }
+
+func (l *Listbox) preferredSize() (width int, height int, err error) {
+	l.lock.Lock()
+	defer l.lock.Unlock()
+
+	width, height = l.sysData.preferredSize()
+	return
+}