fixed performance issues with gradient in Firefox (which also added gradient support for IE9) (FS#2447)

* removed all occurrences of '-moz-linear-gradient'
* added SVG as fallback for Firefox, IE9 and other browsers to page background and buttons

This time PHP and libGD is used so it should work on Windows as well.
The image quality is much better this time and the active highlight
color is read directly from the template's style.ini

This makes it easier to modify or extend the icons in the sprite.
Just place the source files in the pagetools directory and run the build
script.

as discussed at https://github.com/splitbrain/dokuwiki/pull/82#r419699

Add rtl.css as *screen* style, but append all RTL styles with [dir=rtl].
That has the advantage that all styles are in the same CSS output, so there are no different requests.

Later on all styles in rtl.css should be moved to their respective "parent" css file.

made styles for .pad and .site divs more specific to avoid interfering with the same classes somewhere else within the page

removed 'view original' button from new media manager again (was added in b8a84c03) and made a link around the image instead, as that is a more minor change (as it should be during the RC phase) and is what was originally planned

Template authors need to update their _mediamanager.css