Cartesian editor: use text color from system palette.

This fixes the problem that black text was not compatible with a dark
color theme. Because Qt does not provide a suitable color role for
"error text", we still use red for invalid input, but force background
color to maintain legibility.

Change-Id: I230d24a8cce65e76f1346a236260be045d32196d
Minor style cleanup.

Change-Id: I320f24fd81cde9c86ccd1dde94afe65fb59d5618
Merge topic 'cartesian_builder_theme_colors' into master

830704b2 Minor style cleanup.
1ad3c0fb Cartesian editor: use text color from system palette.