Tests for UI tabe

Adds functionality to retrieve an item from the UI table.
Also adds tests for the UI table functionality, including
serialization, label setting, item insertion, and item clearing.

UI/ui_table.hpp

@@ -22,6 +22,7 @@ class UI_table : public UI_item
     std::string column_label(size_t index) const;
 
     void insert_item(size_t row, size_t column, std::string text);
+    std::string item(size_t row, size_t column) const;
 
     /**
      * @brief empty_item This function clears the item and makes it invalid