fix Cancel Create Key Map in MCReader

[alecor450]

In Read Tag, clicking on Cancel after Start Mapping and read tag can take several seconds before the App is available to start another mapping. This is due to the high number of keys being tried in MCReader.

This fix propagates the Cancel request to MCReader and exit the for loops in buildNextKeyMapPart().

Step to reproduce:

• Navigate to the Read Tag Screen
• select all key files
• Click Start Mapping and read tag
• Click Cancel

Old behavior:

Cancel can take several seconds to complete

New behavior:

Cancel resolves immediately

[ikarus23]

Now this is what I call a good contribution/pull request! Good improvement, clean code edit and well documented! Thank you!