Change: Set up suggested widget padding for panel, frame and inset.

Window handlers are free to ignore the suggested padding, however some
handlers have logic that assumes this information was already provided.

src/widget.cpp

@@ -1977,7 +1977,14 @@ void NWidgetBackground::SetupSmallestSize(Window *w, bool init_array)
 				d = maxdim(d, background);
 			}
 			if (this->index >= 0) {
-				static const Dimension padding = {0, 0};
+				/* Setup suggested padding for widgets. */
+				Dimension padding = {0, 0};
+				switch (this->type) {
+					default: NOT_REACHED();
+					case WWT_PANEL: padding = {WD_FRAMERECT_LEFT + WD_FRAMERECT_RIGHT, WD_FRAMERECT_TOP + WD_FRAMERECT_BOTTOM}; break;
+					case WWT_FRAME: padding = {WD_FRAMETEXT_LEFT + WD_FRAMETEXT_RIGHT, WD_FRAMETEXT_TOP + WD_FRAMETEXT_BOTTOM}; break;
+					case WWT_INSET: padding = {WD_INSET_LEFT + WD_INSET_RIGHT, WD_INSET_TOP + WD_BEVEL_BOTTOM}; break;
+				}
 				w->UpdateWidgetSize(this->index, &d, padding, &fill, &resize);
 			}
 		}