Autogenerate IDs in a way consistent with section IDs

[mojavelinux]

This extension autogenerates an ID for each tabset, then uses that value as a prefix to autogenerate an ID for each tab. The ID is necessary in order for the behavior (JavaScript) to correlate the tab's label and content.

The extension is currently using a home-grown strategy to autogenerate the ID. This introduces an inconsistency compared to how IDs for sections are generated. Instead, the extension should delegate to Section.generate_id to generate the ID.

The extension should compute the ID of a tabset as follows:

• the value of idprefix attribute (default _)
• the string literal "tabset"
• the 1-based occurrence of a tabset in the document

The extension should compute the ID of a tab as follows:

• the ID of the tabset
• the value of the idseparator attribute (default _)
• the result of converting label to an ID using Section.generate_id (using the idprefix already established)

By default, the ID of the first tabset is _tabset1. Assuming the first tab in that tabset has the label "First Tab", its ID would be _tabset1_first_tab. If idprefix= and idseparator=-, then the ID of the first tabset is tabset1 and the first tab is tabset1-first-tab.