use imported media files; duotone image colors; extended readme

• chore(docs): examples about shape types in general; about images in detail a8da59c
• chore(test): addElement / modifyElement / duotone for images 399caa0
• refactor(image): unify Xml types; apply modifyElement to images 9bf6111
• fix(types): use proper type for XmlElement c3284e5
• fix(image): await imported files 21421ff
• chore(test): test duotone fill / combination with media load a5f24c4
• chore(test): test loading of external image / update relation target bf7c9eb
• feature(image): load external images / set image target with modifier 47be914
• chore(test): add external png to media dir; add duotone example to SlideWithImages.pptx 3b6744a
• chore(docs): explain selectors and creationIds in detail cbf10ba
• feature(image): modify duotone color overlay for blipFill 6dec687
• chore(shape): remove think cell related contents (wip) 17ad0b5

v0.3.6...v0.4.0