MDL-40939 tex filter: Changes to the TeX notation plugin

- Added configurable setting for path to mimetex binary
- Moved language strings from lang/en/admin.php to lang/en/filter_tex.php
- Config settings now stored in table config_plugins and retrieved
through get_config
- Added upgrade step to move TeX settings to the config_plugins table
and delete settings from the config table

AMOS BEGIN
  MOV [configconvertformat,core_admin],[configconvertformat,filter_tex]
  MOV [convertformat,core_admin],[convertformat,filter_tex]
  MOV [latexpreamble,core_admin],[latexpreamble,filter_tex]
  MOV [latexsettings,core_admin],[latexsettings,filter_tex]
  MOV [pathconvert,core_admin],[pathconvert,filter_tex]
  MOV [pathdvips,core_admin],[pathdvips,filter_tex]
  MOV [pathlatex,core_admin],[pathlatex,filter_tex]
AMOS END