Add helper to set all line bg color in panel.

door.h

@@ -624,6 +624,7 @@ public:
   void update(void);
   door::Goto gotoEnd(void);
   std::unique_ptr<Line> spacer_line(bool single);
+  void lineSetBack(ANSIColor back);
   friend std::ostream &operator<<(std::ostream &os, const Panel &p);
 };
 

panel.cpp

@@ -291,6 +291,17 @@ door::Goto Panel::gotoEnd(void) {
   return door::Goto(col, row);
 }
 
+/**
+ * @brief Set background of all lines in the panel.
+ *
+ * @param back
+ */
+void Panel::lineSetBack(ANSIColor back) {
+  for (auto &line : lines) {
+    line->setColor(back);
+  }
+}
+
 /**
  * @brief Create a spacer line using block drawing characters.
  *