update the edit dialog to remove shortcuts conflicts
display shortcuts conflicts directly in the dialog.

shortcuteditdialog take 2 new optionnal arguments:
* conflictCallback: a function or methode that return a shortcut
conflict name.
* cbArgs: the optional arguments to pass to conflictCallback
add a _num attribute
comment lines that remove the Shift modifier

with its lines when a special character is pressed (é, è, ç), the
equivalent of a QWERTY keyboard is sent.
add "setShortcuts" method
replace "editAction" by "findShortcutConflict" and "removeShortcuts"
add the findShortcutConflict method
set default shortcut to None if it is an empty list
remove unused import
Merge pull request #238 from elmamyra/shortcuts-conflicts

show shortcut conflicts directly in the shortcut dialog