Proposal: Enable 'highlight problematic lines' by default

I propose to enable the 'highlight problematic lines' setting by default. It provides the following advantages:

• It makes errors much easier to spot. On the one hand, the red underlining alone (without 'highlight problematic lines') is sometimes easy to overlook, especially if only a very short part of the line is underlined. On the other hand, with 'highlight problematic lines' enabled, lines with errors also show up in red in the scrollbar minimap.
• It provides a visual indication of the area in which the problem tooltip will show up. This tooltip shows up for the whole line (and right now even possibly for more than that, see: https://invent.kde.org/kde/kdevelop/merge_requests/30 ). With 'highlight problematic lines' enabled, the whole line is highlighted, which matches the tooltip area.

A possible disadvantage is that the line highlighting might be perceived as 'too heavy'. I have been using the setting for a while and personally don't think that this is an issue, but would understand if others see it that way.

Here is an example screenshot showing the setting active:

https://drive.google.com/open?id=1tUVOr4N8RVUltzrprtIWM_fknaq3n6xS

Note: I didn't actually test that the changes in this merge request successfully change the default, since I didn't know which config file I would need to delete to test it.