fdo#70578: writerfilter RTF import: by default style is para style 0

quoth the spec:

"For <style>, both <styledef> and <stylename> are optional; the default
is paragraph style 0."

Of course in order to do that we need to add support for at least
recognizing the \dsN and \tsN keywords to override the default, so that
table styles don't become paragraph styles.

Change-Id: Ic100768581f9e8c327063ff776fbd61ac4242483
n#825305: writerfilter RTF import: override style properties like Word

It would certainly be immediately obvious to any reader of the RTF spec
that \sN will apply the style with index N to the current paragraph.

But actually, that is not what Word does when it reads \sN...
what it really does is to apply the style with index N, and then for
every attribute in that style, apply the same attribute with a default
value to the paragraph, effectively overriding what's in the style.

If that doesn't make any sense to you, well, have you heard the joke
about how many Microsoft engineers it takes to change a light bulb?

Also, \pard apparently implies \s0.

To implement that, change RTFSprms::deduplicate() to recursively
look for style SPRMs that are missing in the properties, and put
in default ones, currently just for 2 keywords \sa and \sb.

This requires changing deduplicate() to be const and return a new value,
since it is no longer idempotent, as the erased SPRMs would get
defaulted on the next run.

While at it, fix RTFValue::equals() which did not compare m_sValue.

This fixes the testParaBottomMargin test that was broken by the fix
for fdo#70578.

Change-Id: I4ced38628d76f6c41b488d608a804883493ff00b