There was (obviously) a copy-paste bug in code converting severities
to GUI's show types. This caused some severities not being
filtered correctly when severity selection in GUI was changed.
Fixes ticket: #3242 (GUI: Unmatched suppression: message in wrong category)
We are using ShowTypes around the GUI codebase so it makes sense
to have it in its own class. And the class also contains related
helper functions instead of scattering them around different
classes.
ShowTypes also contains the visibility settings for all the
GUI severities. Implementation in this commit makes ShowTypes
class to load the visibility settings when it is created. And save
the settings when it is destroyed.
