adding "issues" tab to site-wide search page

i'm now working on a patch for this. i ran into a slight snag w/ a little confusion about menu callback arguments, but i'll be glad to fully understand how that all works, anyway. ;) so, this will be a learning excercise... patch forthcoming, stay tuned.

in case it's not obvious just from looking at the patch, this also gets us a step closer to sanity in the project code regarding the project_issue_page() method. currently, a ton of menu call backs (e.g. for search, subscribe, add, etc) are all filtered through there, and it's doing wonky stuff via arg(), then calling another method to do the real work. there's no need for this anymore. we can just register each menu item to directly call the appropriate methods, and we can use default arguments filled in automatically by the menu system to get the optional parameters (e.g. project name). this way, we don't have to mess with arg() to find the "operation" as well as the project name. eventually, i'm planning to fix this for everything handled in project_issue_page(), and completely remove that function.

also, this patch reformats the menu declarations so the individual attributes are on separate lines, to make things more readable... another thing i'm eventually planning to clean-up in the whole module.

whoops, the previous patch removed a little bit of access checking. previously, you could only do advanced search on a specific project if you had access to view that project (imagine the case where a user has "access own projects" but not "access projects" permissions). new patch restores this behavior...

applied to HEAD and 4.7. backported to 4.6 (since it was so easy). ;)