Render FOFFormHeaderFieldsearchable with class 'btn' when no button class is provided
Save space by only using icons for the buttons of FOFFormHeaderFieldsearchable

Language strings are added as a title
Use tooltips for fieldsearchable
Also use btn-group for Joomla 2.5
Merge pull request #176 from twentronix/improvefieldsearchable

'btn' class assigned to buttons of fieldsearchable by default. Also used icons for search/reset.