Global attribute name standardization for widgets
Better attribute names + other cleanups in widgets
#325

- 1), 2) should be fixed with this update
- 3) should be defined what should happen

Open question:

- I don't know if large and small should be exclusive or.
I guess so, but it was not.

Note:

- I tried to minimize updates to the btnClasses map but that didn't work
I filed a but, see code comment on method `updateBtnClassesJob()`

#786

- I think the button tests is thoroughly.
It can be refined of course if issues occur
or if someone has ideas what can be additionally tested

Note:

- drone.io doesn't install content_shell by default
this has to be done manually (I haven't done it myself yet)
if you add `test/run.sh` to the build script for drone.io the test will probably fail
Improved splitter visual
Implemented scrolling
Cleaned up implementation (scrolling)
Organized a bit for style
Merge remote-tracking branch 'upstream/master' into splitter-css-style
Simplified code
stacking context trick
Temporary commit before merge to get deactivate (overriding activate)
Merge branch 'master' of https://github.com/dart-lang/spark into umop-outline-remember_position
Overriding activate and deactivate to store outline scroll position
Initial setup and implementation of SparkPreferences
Moved CachedPreference and BoolCachedPreference into preferences.dart
Review fixes
initial WIP library
nits
Review fixes
fix the new git tests