Help: Reformat the generator expression documentation.

Use description lists instead of a preformatted block to enumerate
the genex documentation.

Consistently capitalize the description.

Use ``...`` to format genex parameters in the description.

Turn references to commands into links.

Add high level documentation about the motivation for generator
expressions.

Regroup expressions into subsections for different genex types. Add
a high-level description and example of the expressions in each
section.

Explode the documentation for filesystem artifacts, instead of only
referring to the variations.
Genex: Add LOWER_CASE and MAKE_C_IDENTIFIER string manipulators.
QtAutoUic: Handle new -include command line parameter.
QtAutoUic: Add INTERFACE_AUTOUIC_OPTIONS target property.

Transitively consume the property from linked dependents.

Implement configuration-specific support by following the pattern
set out for compile definitions and includes in cmQtAutoGenerators.

Implement support for origin-tracking with CMAKE_DEBUG_TARGET_PROPERTIES.

This is motivated by the needs of KDE, which provides a separate
translation system based on gettext instead of the Qt linguist
translation system. The Qt uic tool provides command line options
for configuring the method used to translate text, and to add an
include directive to the generated file to provide the method.

 http://thread.gmane.org/gmane.comp.kde.devel.frameworks/7930/focus=7992

Implement the interface to provide the uic options as a usage-requirement
on the KI18n target, as designed for KDE.
Merge topic 'INTERFACE_AUTOUIC_OPTIONS' into next

ee18a74 QtAutoUic: Add INTERFACE_AUTOUIC_OPTIONS target property.
640d3e4 QtAutoUic: Handle new -include command line parameter.
8abfe89 Genex: Add LOWER_CASE and MAKE_C_IDENTIFIER string manipulators.
90e98fc Help: Reformat the generator expression documentation.