CMake: Fix KDDockWidgets_PRETTY... option

[ferdnyc]

The PRETTY_QTWIDGETS_HEADERS option was somewhat confusing, as it:

1. Had a KDockWidgets_ prefix, missing the second D

2. Described its purpose as:

   Install DockWidget.h and MainWindow.h as synonyms to the *.h counterparts.

   ...which kind of doesn't make sense.

Fix it by:

1. Renaming from KDockWidgets_PRETTY_QTWIDGETS_HEADERS to KDDockWidgets_PRETTY_QTWIDGETS_HEADERS
2. Checking for builds that have a value set for KDockWidgets_PRETTY_QTWIDGETS_HEADERS but not KDDockWidgets_PRETTY_QTWIDGETS_HEADERS, and migrating that value to the new name
3. Updating the description to be clearer

[iamsergio]

I think I'll remove the option instead. And not have special cases for QtWidgets. It's getting difficult to maintain so many options