Merge branch 'Melanosuchus/gametype_vote' into 'master'

Customizable entries for the gametype voting screen

Adds a system which allow to include customized entries in the gametype voting screen.

These entries are defined by a few cvars:
* sv_vote_gametype_(type)_type : The name of the gametype this entry is based on (eg: dm, tdm, ...)
* sv_vote_gametype_(type)_name : A human-readable name, shown in the voting screen
* sv_vote_gametype_(type)_description : A more detailed description of the gametype, shown in the voting screen

Where (type) is the name of the customized entry.
The icon for custom entries follows the same convention as for regular gametypes,
and the hook sv_vote_gametype_hook_(type) is called as for the real gametypes.

This branch also provides the needed definitions to add **duel** to the voting screen without the need for extra configuration.
(See Xonvote 308 http://xonpickbot.designxenon.com:27500/poll/308)

See merge request !145