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.
Add 'backend' support for the new 'portability' severity. The new
severity is handled, converted to new SHOW_PORTABILITY and added
with correct type to the result view.
There is no menuitem/toolbar button to show/hide portability
items. Likewise there is no line for portability issues in stats-
dialog.
Ticket #2106 (More severities and new xml format)
