Fixed: redundant button CSS

Previously, the LESS stylesheet was generating CSS for buttons using mixins, meaning that the generated stylesheet included many repeated styles. Now, we just create a single 'button' class inside mixins.less and apply it using the 'class' attribute of the HTML tags. This means the 'button' chunk of CSS now only gets loaded once, instead of for each button that uses it.
Fixed: scrolling in fullscreen grid view breaking toolbar layout

Fixes a bug wherein scrolling in fullscreen mode when in grid view moved the (+/-) buttons down. This was happening because the digits were expanding the width of the right toolbar container, pushing the buttons label down.