Allow callers to customise the rendered icon of inplace editable elements

Default element icon

1. Navigate to Courses > Course custom fields in site administration
2. Press Add new category
3. Confirm the Edit category name icon is an editing pencil
4. Edit the value
5. Confirm the edit icon remains an editing pencil

Default element icon for selects

1. Create a new course
2. Add the Activities block to the course
3. Open the blocks side bar
4. Press Forums from the Activities block
5. Confirm the Email digest type icon is a drop down caret
6. Edit the value
7. Confirm the edit icon remains a drop down caret

Custom element icon

1. Edit course/format/classes/base.php method inplace_editable_render_section_name to add a custom icon, changing the return value to the following:

   return new \core\output\inplace_editable('format_' . $this->format, $itemtype, $section->id, $editable,

   $displayvalue, $section->name, $edithint, $editlabel, new \pix_icon('i/calendar', $editlabel));

2. Navigate to your course
3. Turn editing on
4. Confirm the New name for topic icon is a calendar
5. Edit the value
6. Confirm the edit icon remains a calendar