Export: Split placeholders into seperate run

[ValentinFutterer]

Changes
Coloring/resizing replaced text is currently handled naively with #color_green flags. See the replaceInParagraphs method in ÀbstractTemplateExportFunctions. This only works, if [placeholder] is in its own run, which can not be relied upon. We should implement a preprocessing, which splits flagged placeholders into their own run and applies the wanted style change.

Additional notes
Similar preprocessing in #147

Acceptance Criteria
It should be possible to change font, size, color, etc. when replacing a placeholder, without it affecting the surrounding text.