Add dataChanged signal

[T-640]

Whenever I make modifications to binary data inside hex view I need it to emit a signal notifying that this data has been changed. A signal like this inside QHexView class would be ideal:

void dataChanged (QByteArray);

Currently the closest equivalent is documentChanged() signal of QHexDocument, but it is a bit cumbersome to use. It is triggered “spuriously” (for this particular case) by changes in metadata as well, like parts of widget being highlighted.

[T-640]

Added in version 5.0