gui2/ttext_box: Use standard GUI__BACKGROUND_COLOR_* background colors

Note that this widget lacked a background before, making it look like
a simple label with a border and caret. In my opinion, the subtle
background is an improvement that highlights the widget's "dynamicness",
and makes it more obvious when it is disabled.

data/gui/default/widget/text_box_default.cfg

@@ -3,6 +3,18 @@
 ### Definition of a single line text box.
 ###
 
+#define _GUI_DRAW_BACKGROUND COLOR
+	[rectangle]
+		x = 0
+		y = 0
+		w = "(width)"
+		h = "(height)"
+
+		fill_color = {COLOR}
+
+	[/rectangle]
+#enddef
+
 #define _GUI_DRAW_BORDER COLOR
 	[rectangle]
 		x = 0
@@ -85,6 +97,8 @@
 
 			[draw]
 
+				{_GUI_DRAW_BACKGROUND ({GUI__BACKGROUND_COLOR_ENABLED})}
+
 				{_GUI_DRAW_BORDER ({GUI__FONT_COLOR_ENABLED__TITLE}) }
 
 				{_GUI_DRAW_TEXT ({FONT_SIZE}) ({GUI__FONT_COLOR_ENABLED__DEFAULT}) }
@@ -97,6 +111,8 @@
 
 			[draw]
 
+				{_GUI_DRAW_BACKGROUND ({GUI__BACKGROUND_COLOR_DISABLED})}
+
   				{_GUI_DRAW_BORDER ({GUI__FONT_COLOR_DISABLED__TITLE}) }
 
   				{_GUI_DRAW_TEXT ({FONT_SIZE}) ({GUI__FONT_COLOR_DISABLED__DEFAULT}) }
@@ -109,6 +125,8 @@
 
 			[draw]
 
+				{_GUI_DRAW_BACKGROUND ({GUI__BACKGROUND_COLOR_ENABLED})}
+
 				{_GUI_DRAW_BORDER ({GUI__FONT_COLOR_ENABLED__TITLE}) }
 
 				{_GUI_DRAW_TEXT ({FONT_SIZE}) ({GUI__FONT_COLOR_ENABLED__DEFAULT}) }
@@ -137,3 +155,4 @@
 #undef _GUI_DRAW_CURSOR
 #undef _GUI_DRAW_TEXT
 #undef _GUI_DRAW_BORDER
+#undef _GUI_DRAW_BACKGROUND