Add Maps support for TermTreeView

[aboroska]

Display maps as part of the foldable tree in TermTreeView.

TermTreeView can also display Elixir structs now, which are Erlang maps with a special field denoting the name of the struct. The meta field is hidden and the name of the struct is displayed.

Elixir syntax showing a Struct unfolded (default):

Elixir syntax:

Erlang syntax:

LFE syntax:

[andytill]

Thanks!