Help shortcut change allow overwrite

Previously, when adding a conflicting command in Help, VimFx will display an uninformative error dialog. (Then the user needs to find the conflicting key, remove it, add it to new command again)

Now, the help message will be, for example,

```
This shortcut 'c-K' is already used in 'Move current tab to the right'. Do you want to overwrite it?
```

If the user decides to overwrite the key, `c-K` will be removed from the old command and added to the new one.