dwg: add DRW_Mtext

[Rallaz]

For non existent data like background may be first add in dxf style and implement dxf read/write

[TNick]

Following issues need to be addressed:

• rotation angle from X-axis vector (documentation seems uncertain)
• reference rectangle in versions after 2007 (is this relevant at all?)
• drawing direction (find relevant dxf documentation)
• extents(undocumented(?) as width and height of the rotated bounding box)
• the text (need to understand the documentation; seem to indicate a simple counted string but the sample has a 256 count; may be a special code for empty; if the dwgBuffer::getVariableText() is changed to take 256 in consideration the sample MText is parsed fine)
• background needs to be added to DRW_MText: type (none, fill, fill with drawing color), scale factor, color, and transparency
• dwgBuffer::getCMC() needed - issue #20