show shortcut conflicts directly in the shortcut dialog

when there is a shortcut conflict a message with the name of the conflict is displayed in the shortcut dialog.
Is used to configure shortcuts (in preference dialog ) and for editing fragments.
The warning dialog is no longer displayed after validation. I think it is no longer necessary.

for editing fragments:
* if there is a conflict, the conflicting shortcut is now automatically removed.
* if there is an icon, it is displayed.

for the shortcuts configuration:
* remove the default shortcut radio when there is no default shortcut.
* when the default shortcut radio is checked, don't show the shortcut in custom shortcut.

I used a callback system. It is not very clean but I think it is easier to implement. I first tried with events but that was very complicated. maybe you have other ideas?

I don't understand why you use "deleteLater". I have removed them, but there was definitely a reason. I can reuse them.

I removed the editAction method in actioncollectionmanager. I think it is no longer necessary.

I commented lines 158-160 in keysequencewidget because  when a special character is pressed (é, è, ç), the
equivalent of a QWERTY keyboard is sent. it is annoying for example with a french keyboard.

As you can see, my English is very bad and there certainly has syntax errors in the docstrings.

fixes #116