(bug 6100; follow-up to r91315) Being bold and removing $wgBetterDirectionality ...

(bug 6100; follow-up to r91315) Being bold and removing $wgBetterDirectionality (and dependent wfUILang) in core, as most or all work is finished.
Also:
* Introduce classes mw-float-end, mw-float-start so we don't have to use inline css depending on wfUILang()/$wgLang (see HistoryPage and SpecialFileDuplicateSearch)
* Add direction mark to protection log
* Remove specialpageattributes as it is obsoleted by this commit (also fixes bug 28572)
* Add two direction marks in wfSpecialList, which makes ltr links on rtl wiki (and vice versa) display nicely as well (only on those special pages however)
* Revert r91340 partially: use mw-content-ltr/rtl class anyway in shared.css. Both ways have their [dis]advantages...
* Set the direction of input fields by default to the content language direction (except buttons etc.) in shared.css