Add cursor tracking and multiline input navigation

[boxdot]

Cursor tracks a byte position in a text, and additionally a column
and line. It implements navigation of left/right,line up/down,
beginning/end of a line, and basic word operation. Also it supports
unicode and characters of width > 1.

Message input now supports switching between the single line and
multi-line modes via Alt+Enter. Previously, Alt+Enter inserted a new
line. Having a special mode for multi-line is more powerful, since it
allows navigation between lines with the same shortcuts as navigation
in channels and messages. And it allows pasting multi-line text.

CI is switched to use the stable aarch64 rustc compiler. Nightly was
required due to signal-client using the armv8 feature in aes which
was only available on nightly.

Fixes #129

[cs-t1]

Wow that's nice. I'm testing this branch.

I catched a small bug: pressing enter while in search mode does add a newline in the message edition box. I think that's probably not intended.

EDIT: this only happens in multiline mode ofc.

[cs-t1]

Also maybe we should mention in the keybindings cheatsheet that the enter, when input box empty shortcut for opening a URL only works in single-line mode

[boxdot]

@Sup3Legacy Thanks for your feedback. Fixed.

[cs-t1]

This new interface is really a nice experience. Maybe you can also refine the search mechanism if you see any change you'd like to make.