[svn] Added SVN clean up dialog.

Dialog executes 'svn cleanup' command for a user picked directory.
This action is available only for directories.
Added functionality:
- delete unversioned files and folders;
- delete ignored files and folders;
- include externals;
- user can change a directory in a dialog itself by typing it or by
picking it;
- added checks if a directory is valid: for invalid paths "OK" button is
disabled.