[sailfish] Made the content teaser optional

Closes #101 on sailfish
[sailfish] Renamed setting teaseContent to showExcerpt

Following the tt-rss naming.  Regards #101
[sailfish] Reorganized Settings Dialog

- Grouping according to pages
- Unify naming (from the users' pov; the variable naming is still a
  bit chaotic)
Ordered settings.hh and settings.cpp
[sailfish] Switched from ComboBoxList to ComboBox in Settings

Removed unused code from ComboBoxList, too.
Fixed return type
[sailfish] Let the user define the length of title & excerpt

Closes #102 on sailfish
[sailfish] Use SectionHeader instead of Label
Merge pull request #109 from michael-k/settings

Settings