Item-Memory: Allow resolving memory-keys in INI

[pjotrek-b]

Thanks to CIItem::$memory, it's already possible to exchange information between tasks.
Currently, it's only possible to read/write Item-memory from within the code (CIItem::remember(), CIItem::recall()) - not from within the INI.

The following example shall be possible:

• One task extracts the DAR (Display Aspect Ratio) from a metadata file (like 'source_dar=4:3`)
• FFmpeg task uses this value to write /that/ DAR into a created video container file:
  `ffmpeg -i ... -aspect [@IM.SOURCE_DAR@]

The idea is to:

• prepend some prefix to memory-keys (like memory keyname goes to [@IM.KEYNAME@])
• resolve all existing memory-keys as [@Placeholder@] variables, everytime a new memory-value has changed, or a new memory-key was added.