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
style multiparts like other exercises

i.e. without the boxiness
hide "click to add part" on mouseleave

space-saving measure same as it's done for regular exercises
make blue droppable area less tall

plus a little margin so it doesn't look too awkwardly close to the
elements it'll be between.
remove excessive and redundant min-heights
for tables, preview insertion spot with arrow instead of circle

This preview also creates a new line that goes away if table insertion
is cancelled (the previous preview implied it might add the table inline
to the current line).
Merge remote-tracking branch 'origin/58657378-boxes-and-spacing-2nd-attempt' into testing-new
Merge remote-tracking branch 'origin/57878870-arrow-instead-of-circle' into testing-new