[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.