Introducing front end/back end option

Also added a note field for more info
Moved the field mode (functionality) after the skins

Added/ modified
PLG_TINY_FIELD_SKIN_ADMIN_DESC="Select skin for the Administrator Backend interface"
PLG_TINY_FIELD_SKIN_ADMIN_LABEL="Administrator Skin"
PLG_TINY_FIELD_SKIN_DESC="Select skin for the Frontend interface"
PLG_TINY_FIELD_SKIN_INFO_DESC="Copy your new skins at: /media/editors/tinymce/skins"
PLG_TINY_FIELD_SKIN_INFO_LABEL="For customized skins go to: skins.tinymce.com"
PLG_TINY_FIELD_SKIN_LABEL="Site Skin"