Fixed numerous warnings generated by Java and Lint.

Added baselineAligned option to main_activity.xml as suggested by Lint to increase performance.
Fixed spelling error in values-de/strings.xml (this maybe should be done on transifex?)
Removed several unused strings from values/strings.xml
Added suppression for Lint warning to ClementineExceptionHandler.java since hard-coding the date here is fine for debugging purposes.
Removed unused imports from ClementineLibraryDownloader.java, ConnectDialog.java and Security.java
Added warning suppression to ClementineSettings.java since deprecated methods will be removed during later refactoring.
Added locale-specific method calls to FileDialog.java to fix potential locale-specific bugs.
Added serialVersionUID to IabException.java to remove java warning about serializable interfaces.