remove boxy styling from definitions

style it more like an exercise / as in mock-up / as in cnx
make "copy element" option less conspicuous
improve styling of x and gear

make them lighter since i apparently can't make them thinner :-(
better spacing (centered and without extra space above/below)
hover behavior more like mock-up
update styling of quotations

use left/right quote mark images instead of generic box to style
quotations
remove "extra" padding around figures
transition height for solution-controls too ...

... so that exercises don't take up as much space when not hovered