qdoc: Revised logic for which example file to open in Qt Creator

qdoc stores the 'files to open' into the example manifest. These
files are opened in Qt Creator's editor when an example is selected
from the Welcome / Examples list.

This change uses the following criteria (case insensitive), in
order of preference:
    - .qml file matching the project name
    - .cpp file matching the project name
    - .h file matching the project name
    - main.qml
    - main.cpp

A 'mainFile = "true"' argument is written for the file that is
preferred to be the top-most file.

Having a main.qml file take precedence over main.cpp ensures that
most Qt Quick examples open into the relevant QML code instead of
the boilerplate C++ used for launching the application.

Task-number: QTBUG-37203
Change-Id: I2ea58a31b1284f4f7d424dd35d49a84a23a88c23
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>