Improve XmppClient GUIMessage struct handiness

[elexis]

The lobby xmpp code sends messages to JS via custom struct. But that is bad since it hardcodes a set of possible arguments. Some examples:

• If someone changes the nickname, the old nick will be sent in text and the new name will be sent in data, entirely undescriptive
• If someone is kicked (​https://code.wildfiregames.com/D116), the nickname will be sent in text and the disconnect reason in the data` field

The custom struct is then parsed to a JS::MutableHandleValue in GuiPollMessage.

Instead we should create those JS values directly, thus allowing to easily add further arguments. It would also allow merging the "kick" and "ban" case and just pass another boolean.

The code will then appear comparable to CNetClient::GuiPoll and PushGuiMessage.

[elexis]

It would becmoe a std::deque<JS::Heap<JS::Value>> m_GuiMessageQueue just like in the NetClient.h.

[elexis]

In 20064:

XmppClient cleanup.

Allow XmppClient to use arbitrary property names in GUIMessages.
Remove duplication by calling CreateGUIMessage when receiving chat messages.
Inline PushGUINotification.
Use std::string in the GUIMessage because every data source is of that type.
Apply the wstring_from_utf8 conversion to ensure correct display of less common UTF characters instead in the GuiPollMessage method.
Add room subject change chat message.

Differential Revision: ​https://code.wildfiregames.com/D835
Refs #4482
Comments by fpre, Vladislav and leper

[elexis]

Works for now. Creating a custom ScriptInterface by itself can cause trouble and isn't necessary or an enhancement.

[elexis]

Since we need a third property in at least one case, the ScriptInterface approach might be nicer, see #4877.

[elexis]

In r22856:

Support creating lobby GUI messages with arbitrary arguments instead of forcing every message type into the same struct type, refs #4877 / rP19514 / D339.

Creates the GUI messages directly as JS::Values and removes the intermediary rigid struct. Implementation similar to ScriptInterface::CreateObject? from D2080.

Differential Revision: ​​https://code.wildfiregames.com/D2264
Tested on: clang 8.0.1, Jenkins

[elexis]

In 22859:

Fix wrong "player is not a moderator anymore" lobby chat message following initial implementation in rP19514/D339, fixes #4877.

It needs to test against the historic role and not the current role, similar to rP16997, refs #3386.
Implements the third GUI message property using the arbitrary lobby GUI message patch in rP22856/D2264, refs #4482, rP22855/D2265.

Differential Revision: ​https://code.wildfiregames.com/D2266
Tested on: clang 8.0.1

[elexis]

In 22880:

Pass XmppClient CertificateErrorToString independent of ConnectionErrorToString to JS using D2264 / rP22856, refs #4482.

This also allows making the enum to string functions static, refs D2271.

Differential Revision: ​https://code.wildfiregames.com/D2274
Tested on: clang 8.0.1, Jenkins