Add plugin utilit functions and a decorator.

addToGroupContextMenu is almost identical to addToLeafContextMenu.

getSelectedLeaf - returns selected pytables object, usefull when used
with context menu action

long_action is a decorator that chages the cursor to the wait cursor
and also optionally dispays message in the status bar, usefull for
longer actions
Merge pull request #20 from alexey-naydenov/plugin_utils

Some more functions that are usefull for plugins.