Use a XML stylesheet (xslt) for generating the `styles.xml` for ODF-Files.
Generate ODF-style for custom styles in map.
Add display-name for style and replace space by underscore in name.
Add a todo-comment.
Fix tests for bold and italic font-style.
Fix: Size needs to contain a unit (`pt`).
Add color attribute.
Reformat comments to make them readable.
Unify the code for emmitting a node.

As a side-effect this solves a minor issue: sytles names have had lots
of leading and traling white-space.
Cleanup: Remove unnecessary <otherwise><choose> constructs.

<choose> only executes the *first* <when>-clause where the test is
true. So there is no need for <otherwise><choose> constructs.
Reindent.

I did not to this in the last commit to keep that one clear to
only removing the <otherwise><choose> constructs.
Introduce a new template for output of headings.
Only format "AutomaticLayout" leveled nodes as headings.
Honor automatic layout modes (both non-leaf-mode and all nodes).

Styles are applied to nodes up to level 4. Automatic layout mode
overrules nodes styles, as it does in the map.
Take heading level from @LOCALIZED_STYLE_REF instead of $depth.
Introduce a new template for output of paragraph-styles.
Put Freeplanes pre-defined styles into `styles.xml`, too.

Style-names are not yet localized nor capitalized.
Honor pre-defined styles.
Honor custom styles.
Update a comment.