Variable footer, subfooter and subheader height

Adding sass variables for footers, subfooters and subheaders with different height, in reference to issue #2334 reported by me a month ago

Sass variables added:
 - $bar-footer-height:               44px !default;
 - $bar-subheader-height:            44px !default;
 - $bar-subfooter-height:            44px !default;