Refactoring: prepare to filter options based on selected authors
Use shared property instead of parameter for selected-author-check
Rename count-author-records to count-total-authors

This module will now be responsible for publishing the property
"total-authors" with the count, while a separate module, named
display-total-authors will display the value in the DOM.
Extract display-total-authors from count-total-authors

The new module uses the property published by the existing module
to update the display.
Rename count-selected-authors to count-total-authors-selected
Rename count-selected-authors-percentage

The new name, display-total-authors-selected, reflects the new
responsibility: this module will update the display of the total
and percentage of selected authors, while the module named
count-total-authors-selected will publish properties with the
computed values.
Count selected authors in one module, update display in another
Use new property "total-authors" instead of a parameter

Also, the value is no longer computed here, only retrieved.
Prepare to update the counts displayed in each filter option

This is an intermediate step towards the display of dynamic values
in each option of filters: first the total of authors of the category
currently selected, next the total of authors in the category.

Two separate functions, one to fill the list initially, one to
update the list when the set of selected authors changes, will
share a set of utility functions to prepare the display.

Another feature expected is the filtering of options displayed
to remove options which match no author in current selection.
Prepare utility functions to compute totals for each category

I also moved the code to update the display of the total categories,
in which I plan to display the number of categories selected rather
than the total number of categories.
Use DOM element retrieved by id rather than navigating the DOM again
Display the total number of categories selected

The categories with no author or no author selected
are now omitted from the total displayed after each
filter list.
Remove 1 row from offset given to callback in forEachData

Since the headers row is omitted, it should not be counted
in the offset either. As a result, the record offsets now
start at 0 rather than 1 in the callback.
Compute isFirstOption from offset provided in callback
Display total authors of the category in each option

This total replaces the total of all authors, always the same,
displayed in all options so far.
Replace getExtraText() with getFullText()

I also removed the local variable extraText and the logic for
the selection of totals depending on the category, now present
in getTotalCategoryAuthorsSelected() and getTotalCategoryAuthors().
Move setOptionText() to form-utils

I also generalized the function to handle both the cases when
a text node is present in the option or not. Note that the whole
function could be simplified using innerHTML if needed.
Extract setFullText() from fillFilterSelectionList()
Extract function displayTotalCategoriesSelected()
Remove unused variables