Refactor all view types as separate modules + view type dependencies & settings

[JoryHogeveen]

Currently the users, roles and capability views are always loaded.
Not everyone is using all these features. Though it will be a minor improvement, lets make all these features optional.

• Refactor all view types. (v1.8)
• Enabled by default: Users, Roles, Capabilities and Languages.
• Store enabled modules in the global option. No need to separate option keys.
• Add dependencies
  • Role Manager is depending on the Capabilities view (mabe refactor this?)
  • Role Defaults is depending on the Roles view.