Issue #1165962: Generalize translation sets. Reimplemented taxonomy translation set overview, added set delete

Issue #1164922 by webflo | neuronick: Fixed Undefined variable $reset in i18n_menu_translated_tree().

Patch was already committed in e801d538

This reverts commit 3c985cbd.

Issue #1155276 by zilverdistel: Changed Function i18n_node_type_enabled() should call locale_multilingual_node_type()(()).

Issue #1157484 by das-peter | skilip: Fixed Both delete translation sets when deleting a menu and when deleting menu items.

Issue #1157456 by skilip: Changed Remove the  'Language neutral' option when translation mode is set to I18N_MODE_LANGUAGE.

Issue #1157848 by tobiasb: Fixed missing menu callback for the edit form of the translation set (menu link).

Issue #1158150 by tobiasb: Fixed wrong path for the edit form of the translation set (taxonomy term).

Issue #1157420 by skilipand myself: Copy the workflow of taxonomy term translation to menu item translation.

Issue #1157382 by skilip, dereine and myself: Improve and unify multilingual descriptions on vocabulary and menu forms

Issue #1157826 by dereine: Create a way to write a translation/localization string into the database

Issue #1070256 by idflood, webflo | teslegch: Fixed Localize terms show always the default language.

Followup to #1157458 by dereine, webflo: Fixed 'Taxonomy description is not translated on the taxonomy node listing.' Add Test for translated term description.

Issue #1157456 by skilip: Remove the 'Language neutral' option when translation mode is set to I18N_MODE_LANGUAGE