Use matrix-sdk-ui for internal representation

[VAWVAW]

Use matrix-sdk-ui for internal statekeeping.
This will reduce inconsistencies with other clients by respecting the spec better (e.g. message order by homerserver order instead of origin server timestamp or which messages should get a read receipt).

Things this PR provides/does:

• update modalkit to newest git
• update matrix-sdk from 0.14.0 to 0.16.0
• make sending nonblocking
• implement :invites list
• show replied to message for replies to old messages that are not loaded
• show and update the fully_read marker

Future Possibilities (that are way easier with matrix-sdk-ui):

• support polls
• show the raw json for an event (in combination with Add ability to view info about a message #566)
• differentiate in ui between unread messages, notifications and highlights (maybe with Add color config #472)
• show a shield state for problematic messages
• going directly to the message context on :replied instead of paginating backwards until we hit it

includes #464
provides #445, #516, #579
fixes #588