OT-717: Unreadable Status Bar

[phranck]

Fixes #449

BUGFIX

• Fixes a bug that happens on switching between Light and Dark mode, which results in an unreadable system status bar

ADDITIONS & CHANGES

• Adds a new ThemeKey enum value system
• Changes setting for Light/Dark mode in user profile by removing the switch
• Adds a new screen for switching application appearance to Light/Dark or System
• Refactors all ThemeKey enum values from UPPERCASE to lowercase
• Adds vibrateLight method for haptic feedback when user changes appearance
• Adds new assets for all three theme states: Light, Dark, System
• Adds method extension with a getter stringValue for enum ThemeKey to get a string of the enum value only
• Cleans up code

ADDITIONAL NOTES

• Adds activeColor setting to all used Switch.adaptive widgets with the value of CustomTheme.of(context).accent, because on iOS they had the default green color which doesn't fit our application color theme (unrelated regarding the topic of the ticket).

---

Removed after first discussion

• Adds Preference class with just two static methods set({@required value, @required String forKey}) and value({@required String forKey}) for convenience and better readability
• Adds static getter/setter on Preference class for easier handling of preference values and better readability (NOTE: Should be discussed if we adopt this system overall)
  So instead of calling:

  setPreference(preferenceAppThemeKey, describeEnum(ThemeKey.light));
  

  we can call:

  Preference.themeKey = ThemeKey.light.stringValue;
  

[florianhaar]

After setting the appearance to "system" the status bar is completely dark. Light and dark work perfect. See attached image:

[phranck]

After setting the appearance to "system" the status bar is completely dark. Light and dark work perfect. See attached image:

Oh damn... this is a regression. Thanks!

[phranck]

@florianhaar @Boehrsi
Triggered by this Flutter issue I tried the Flutter beta channel and tested again. Et voilá - it works as expected on both Android and iOS!

[Boehrsi]

Notes / stuff to check tomorrow for /me:

• Setting the accent color on numerous elements shouldn't be required as this should normally be handled by the global theme
• Dimensions file: 6dp value should be a specific value not a default
• Check theme logic and compare with system and set something depending on this seems to be quite often in there. Possible to simplify in 1-2 methods?
• Show all team members the "reduced boiler plate event / state handling" (context: lib/src/settings/settings_appearance_event_state.dart)

[Boehrsi]

LGTM, will check functionality and the framework problem tomorrow morning. Also have some questions / ideas we can talk about tomorrow (see my previous comment).