recognize t.me proxy qr codes

[r10s]

this PR adds the type DC_QR_SOCKS5_PROXY to dc_check_qr() for supporting telegram proxy QR codes. if returned, the UI should ask the user if they want to us the proxy and call dc_set_config_from_qr(); afterwards (plus maybe dc_configure()).

idea is to improve our proxy story, follow ups may be:

• in UI, - move proxy out of "Account & Password", as a separate "Proxy Activity" (it should stay in "Advanced" for now, however, below "Server", which might be moved up)

• allow opening the "Proxy Activity" from the welcome screens three-dot-menu (that would also solve a long standing issue that entering the email address bypasses the proxy

• show proxy usage in the "Connectivity View" and/or add an icon to the main chatlist screen (beside three-dot menu) in case some proxy is in use; tapping this icon will open the "Proxy Activity"

• the the new "Proxy Activity", add a share / show proxy QR code button. that would generate invite links in the form https://i.delta.chat/socks#... - so that tapping then opens the app. support for these links need to be added to core then.

• handle a list of proxies in core, offer selection in UI. the list could be one for all profiles and could be filled eg. by normal invite links or other channels

[iequidoo]

I meant the following "solution" for now, to not complicate the code with transactions or so. Anyway, this is minor.

Suggested change

	// disable proxy before changing settings to not use a combination of old and new
	context
	.set_config_bool(Config::Socks5Enabled, false)
	.await?;
	context.set_config(Config::Socks5Host, Some(&host)).await?;
	context
	.set_config_u32(Config::Socks5Port, u32::from(port))
	.await?;
	context.set_config(Config::Socks5User, None).await?;
	context.set_config(Config::Socks5Password, None).await?;
	context.set_config_bool(Config::Socks5Enabled, true).await?;
	// invalidate proxy before changing settings to not use a combination of old and new
	context.set_config(Config::Socks5Host, None).await?;
	context
	.set_config_u32(Config::Socks5Port, u32::from(port))
	.await?;
	context.set_config(Config::Socks5User, None).await?;
	context.set_config(Config::Socks5Password, None).await?;
	context.set_config(Config::Socks5Host, Some(&host)).await?;
	context.set_config_bool(Config::Socks5Enabled, true).await?;

[r10s]

i added a commit to support the parameters &user= and &pass in t.me/socks urls