drawingML export: gradient fill defined by imported theme

Plus avoid writing out empty <a:gradFill></a:gradFill>.

Change-Id: I50fa92b2cc30eedfe28a8c6af697d43d98a3aee8
drawingML import: run fonts inside group shape

Change-Id: Ib54fe868c11c0c22b85d8821fa7274c80918362c
DOCX export tests: increase strength of testing nested text frames

Change-Id: I370f22ee948f5a760ba01c98a8cfb5fac96f98b7