in current layout the filter-toggle button is placed in the middle of the page and eats some part of page height. This is a pure javascript functionality, so it'll be much easy to support it into js as much as possible.

with suggested patch:
1) button is moved to controls (more space for important info)
2) no php processing for the button (less server load, button will be added by js if filter-space presents in page)

changed files:
modified: img/icon-sprite.svg (+ 2 icons)
modified: include/classes/html/CFilter.php (- unused code)
modified: include/defines.inc.php (- unused defines)
modified: js/gtlc.js (+ update scrollbar time on filter show, if page was loaded with hidden filter)
modified: js/init.js (+ btn-filter functionality)
modified: styles/blue-theme.css (cleaning)
modified: styles/dark-theme.css (cleaning)

two page views attached (on/off filter)