DocBook: output inline equations in their own paragraph if they are the only inset...

DocBook: output inline equations in their own paragraph if they are the only inset of the paragraph.

It's invalid to have an <inlineequation> outside a paragraph. Another solution would have been to change InsetMathHull::docbook to generate an <informalequation>, but that function would have required more knowledge about its context than now.

Need more tests to determine if <inlineequation> should only be output for hullSimple or in more cases like:

		const static std::set<HullType> inlineHulls = {
				hullUnknown,
				hullNone,
				hullSimple,
				hullEquation,
				hullMultline,
				hullGather,
				hullRegexp
		};