Automatic tab number should respect document order

[mojavelinux]

When generating an ID for the tabs block (which also gets used to generate IDs for its child elements), the tab number used in the ID should follow the document order. Currently, if a tabs block is nested in another tab, the tab numbers become out of order. This happens since the children of a tabs block get parsed before the ID is generated for that tabs block. This can be corrected by reserving the tab number for the tabs block before parsing the children. In the event the extension does not return a tabs block, it must reset the tab number to the previous value (since it should not occupy a tab number).