This partially reverts commit 891cf4ac0c
which set `sans-serif` as the default font.
Setting it back to Arial solves SVG export and rendering issues,
especially on platforms (Windows, Mac) where the default font
metrics differ highly from Arial.
NOTE: The font can still be easily overriden if integrators wish
to do so in order to give their users a more native look and feel.
Closes#819
* take numeric line height into account when
rendering text labels
* take line height into account when directly
editing labels
* use default line height of 1.2 for text rendering
Closes#803