Add API to enable/disable accounts in account manager

[link2xt]

In account manager it should be possible to disable accounts in such a way that they are not started when dc_accounts_start_io() is called.

It could be implemented as dc_accounts_enable(account_id: u32, enabled: bool) and dc_accounts_is_account_enabled(account_id: u32) -> bool calls.

Whether account is enabled or not should be written into accounts.toml. #4295 should be fixed before tackling this issue, because otherwise each toggle of account status may fail and corrupt accounts.toml.

[link2xt]

Desktop currently has implemented an UI hack that disables background synchronization for all accounts, and this feature is about implementing a per-account switch telling whether the account should be synchronized in background or not.

[link2xt]

Draft PR: #5314