[optim] cache rset column structures When iterating over large resultsets (lot of colcumns, lot of lines) to build entities, caching column structure (which attribute or relation is stored in each column) can improve performance noticeably.

[js] make _loadAjaxHtmlHead aware of mod_concat-like urls _loadAjaxHtmlHead keeps track of already imported resources (js / css) to make sure they're not included twice, but with mod_concat-like urls, things get a bit more complicated : if BASE_URL/data??foo.js,bar.js is already imported, then loading : BASE_URL/data??bar.js,spam.js should only load spam.js

[etwist] in debug-mode, concat-file is rebuilt if one of the concatenated file is more recent