Fix notebook tab switching with keyboard if some pages are hidden

When switching the current notebook tab, we need to take only visible
pages into account.  If we don't and we try switching to an invisible
page, nothing happens.

In practice, the issue is visible on the message window notebook if one
of the tabs are hidden due to one of the "msgwin_*_visible" settings.
Mark Geany.txt with filetype

Since we have a reStructuredText filetype, mark doc file so it
automatically opens with the right filetype.