PPT: Improved processing of formatting properties.

* PPT has 7 different text types, each has 5 official levels. Some of them inherit formatting
properties from the masters hierarchy (some start with default values). In case the master
doesn't provide a property, we have to check either properties for a related text type or
check properties for the previous levels. Sometimes a combination of both helps. Fine tuning
is still required on test files.

* Text of type Tx_TYPE_OTHER should have most of its formatting stored in the TextContainer.
Any missing property must be looked up it the hierarchy of defaults and its levels. It does
not inherit from the masters hierarchy.

* PPT uses font-independent-line-spacing, which means that line height is calculated only from
the font height as specified by the font size properties. Then a few approaches are used to
specify the margin-top and margin-top attributes. It took me a while to figure out the logic
based on a bunch of test files. Checked both OOo and the ODF plugin from MS Office 2007, non
of those does it all correct.

* Also changed the structure and cleaned up the pptstyle source file in order to add the missing
logic.