Paginate every second page

[oliver-stoehr]

This pull request adds a new option to the pagination mode menu.
"Sheet counting with empty page"/"Blattzählung mit Leerseite" allows a pagination with a counter increased for every second page while alternating between the counter and "UNCOUNTED".
This produces a pagination like this:
1, UNCOUNTED, 2, UNCOUNTED, 3, UNCOUNTED, …

This mode is also compatible with:
roman (I, UNCOUNTED, II, UNCOUNTED, …) and
alphabetic (a, UNCOUNTED, b, UNCOUNTED, …) pagination.

Technical base for this new mode is a String like this:
¿UNCOUNTED¡1½
It can also be used in the advanced pagination (The 1 can be replaced with a different start value, also using a start value for roman or alphabetic pagination, like I or ´a´).

To be able to output the counter conditionally on every second page, a code adjustment was necessary.
This adjustment has one constraint for advanced pagination:
The characters following ¿ and ¡ are no longer interpreted as free text (implicitly!).
For example, using a string like ¿v to output v for every second page now requires (explicit) escaping to prevent an interpretation as roman numeral: ¿`v`.
(That means configurations like recto verso pagination change, e.g. from 1°¡r¿v½ to 1°¡r¿`v`½.)

This only affects the advanced pagination. Additional escaping is only necessary for characters that should be free text, but could be interpreted as roman numerals or decimals (I, i, V, v, X, x, 1, 2, 3, …).

More details on why this is necessary

Generally, it is necessary to escape free text with back ticks (`) to prevent unwanted evaluation.
This was not necessary for text following ¿ or ¡ (e.g. in 1°¡r¿v½ for recto verso pagination = 1r, 1v, 2r, 2v, …), as text following ¿ and ¡ was implicitly interpreted as free text.

To output the counter on every second page, a String like ¡1 would be necessary, but did not work in this combination, as ¡ prevented the interpretation of 1 as counter).

This implicit interpretation as free text is removed in this PR, as it prevented any conditional output of a counter.
This change is only relevant to users of the advanced pagination, as all other counting modes like "Recto Verso Pagination" have already been adjusted accordingly in this PR.

Resolves #6457.

[oliver-stoehr]

@andre-hohmann would you mind taking a look at my description of this change?
The change has one constraint for the advanced pagination as described above. I would like to get your opinion on whether this constraint might be critical for users of the advanced pagination.

[andre-hohmann]

@oliver-stoehr : i am sorry, but i cannot assess the impact of the constraint. I do not apply the advanced search.

Maybe @matthias-ronge can assess it. As far as i know, he added the advanced search in the old documentation.

I will also inform the Community Board.