Remove some empty table rows and columns from the Glade file

Cleanup some useless placeholders, and replace some one-column tables
with vertical boxes.

No UI changes.
Fix an inverted check
Properly mirror document_get_from_page() in document_get_notebook_page()

Now document_get_from_page() supports arbitrary nesting inside the
notebook page, update document_get_notebook_page() to support it also,
by searching up for the direct notebook child rather than assuming it
is the ScintillaWidget's direct parent.
Avoid code duplication in document_get_current()
Fix a few missing/extra includes
infobars: Use wrap labels to avoid cropped infobars

Since the infobar is backed in a GtkPaned that allows "shrink", if the
content is too wide, it's far end will just get cropped.  Also, wrap
labels are nicer to display possibly long texts as it flows naturally.