removed all browser-specific gradients as the recently (in 42ff6730) introduced svg makes them unnecessary

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

This removes the cachekey parameter again and instead follows
@michitux's suggestion to trigger an event for the cache usage