fix: account settings not restoring on "Cancel"

[WofWca]

...if the user attempted to set invalid
credentials by pressing "OK" prior to pressing "Cancel"

TODO:

• This is based on fix: EditAccountAndPassword not showing error #4032. Merge that first.
• This issue is also present on DC Android. Make an issue before we forget?
• This means that there are users for whom the "Password and Account" dialog shows credentials that are actually invalid. This commit does not address this, neither does fix: EditAccountAndPassword not showing error #4032
  Make an issue?

But there are still cases where invalid credentials are still stored,
see comments.

[Simon-Laux]

Isn't that sth that core should do? the variables it actually uses to connect are prefixed with configured_. I think we should rather restore from those.

[WofWca]

To be honest, I don't know much about the core and its API.
But yeah, it sounds weird that you can commit invalid credentials and then it keeps working on the old credentials while still showing the invalid ones.

I made this merge request with an assumption that the core API doesn't need to be changed.

[Simon-Laux]

maybe we could ask core for an api that we can call when the user closes the windows to reset it, or have it behind a reset button in that change credentials dialog.
like the core api should reset the vars to the ones that are actually used?
CC @r10s @link2xt @iequidoo