factored out more code around requesting tweet pages from derived fragment classes into base class, and moved view handling out of onActivityCreated and into onCreateView
removed unused settings actions from compose and profile activities
added FRAGMENT_NAME constants for timeline fragment classes
use FRAGMENT_NAME constant rather than a magic string to determine which type of fragment to instantiate
added ProgressBarManager class to keep track of active network requests and enable/disable progress bars
setup timeline activities and fragments to use ProgressBarManager