Remove unused page from stacked widget form, move duplicated code
into utility functions, reduce state variables and string comparisons
in RssFetcher, clean out includes. Give WelcomeModeTreeWidget
uniform row height.
This removes the all hard dependencies to and from welcome screen,
except the one to the core plugin. More in detail:
- Add IWelcomePage to add a tab to the welcome screen
- Move tabs in the modules where they belong
- Enables QHelpManager to open help fullscreen and contextually
- "Getting Started" moves to Qt4ProjectManager
- Projects & Sessions (aka "Develop") moves to ProjectExplorer
- "Community" remains in the welcome plugin for simplicity
