AECD: Incorporate AECD among the aggregated itwêwina dictionary sources

[aarppe]

Add AECD among the three possible dictionary sources for itwêwina. Thus, we'll need the following steps (at least):

• [x] AECD implemented as a dictionary code, with a pop-up expanding on this source
• [x] AECD as an optional source in the settings
• [x] #101
• [x] #103
• [x] #104.

[aarppe]

@nienna73 Created this meta-issue for co-ordinating the aggregation of AECD. Due to Daniel's speed, we now know that we could "easily" aggregate close to 75% of the AECD content, and that is sufficient for us to proceed, while we'll sort out what to do with the remaining 25%.

[aarppe]

The first versions of all the subtasks above have been implemented, resulting in some follow-up tasks, e.g. #105.

[fbanados]

This is almost done, but in the aecd/ branch. Once we merge this could be closed.