Fix overdraw issues in the conversation view

Remove the redundant window background and row backgrounds to improve
drawing performance.
Merge pull request #992 from veeti/overdraw

Fix overdraw issues in the conversation view
Switch from classic context menu to ActionBar in the conversation view.
Just a replacement following the Holo style guidelines. Not implementing multiple selections.
Removed one unneccessary instance variable.
Merge pullrequest #896 for issue #577.