This patch resolves the responsiveness issues for the toolbar in the
viewer. Depending on the language (for example the Dutch language),
elements could overlap when the viewport size is reduced.
The main issue here is that the CSS rules are unnecessarily complex and
handle lots of different cases (LTR/RTL, displacements for specific
viewport widths, et cetera). By removing this complexity and letting the
browser handle the responsiveness, we not only get simpler CSS rules and
HTML mark-up, but the responsiveness issues are mostly fixed at the same
time. We no longer have to position the elements manually (by setting
their `left` attribute value) anymore.
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