doxygen docs: fix insufficiently namespaced group names

These led to clashes, since group names must be globally unique.

src/lib/elementary/elm_code_diff_widget.h

@@ -12,7 +12,7 @@ extern "C" {
 
 /**
  * @brief UI Loading functions.
- * @defgroup Init  Creating a diff widget to render an Elm Code backend
+ * @defgroup Elm_Code_Diff_Init  Creating a diff widget to render an Elm Code backend
  *  when it's referencing a diff file
  *
  * @{

src/lib/elementary/elm_code_line.h

@@ -40,7 +40,7 @@ EAPI void elm_code_line_free(Elm_Code_Line *line);
 
 /**
  * @brief Line manipulation functions.
- * @defgroup Content Elementary Code Line
+ * @defgroup Elm_Code_Line_Content Elementary Code Line
  * @{
  *
  * Functions for changing the content of lines in an Elm_Code_File

src/lib/elementary/elm_code_text.h

@@ -14,7 +14,7 @@ extern "C" {
 
 /**
  * @brief Line text handling functions.
- * @defgroup Text access and manipulation within lines
+ * @defgroup Elm_Code_Text access and manipulation within lines
  *
  * @{
  *