Fix menu bar layout to prevent invisible items

Not ideal yet: ScrollPane is positioned absolutely at 141, so now it will get obscured by the dark menu bar. Instead, the menu bar should get its required height, and ScrollPane should get the remaining page height.
Add wrapper element 'all-headers' around all header divs
Set height with JavaScript, rather than use absolute positioning
Add wrapper element button-panel to buttons
Add padding to resemble original layout
Take ScrollPane padding into account when setting height
Also tweak layout on window resize
Comment + Remove obsolete property
Separate setting height from tweaking layout + Comment
Merge pull request #10 from AmpersandTarski/fix-menu-bar-layout

No more invisible menu items!