#23778 Improved QT Styling
Adds a new method to the engine base class:
_initialize_dark_look_and_feel() which replaces the
_get_standard_qt_stylesheet() method (which will still be around however
deprecated).

This method sets up a visual environment very similar to the one in
maya. It is using a slightly different approach than just using a
stylesheet - instead, it is a combination of style, palette and some css
on top. The hope is that this should provide a broad, solid and
consistent style base for all apps and widgets and ensure that they are
looking similar to the way they would look in Maya (or Nuke for that
matter).

Engine initialization will typically look something like this with the
new method:

# start up our QApp now
qt_application = QtGui.QApplication([])
qt_application.setWindowIcon(QtGui.QIcon(self.icon_256))
self._initialize_dark_look_and_feel()

Closes #80.
#25657 include statement now supportes environment variable expansion.
This allows paths declared via the include or includes statements in 
the templates.yml or environment files to contain environment variables.
#25435 Better resolve of hooks with {engine_name}

If a hook is a new-style hook and has a default value which contains the
{engine_name} token, the code now checks to ensure that an
implementation actually exists for the current engine.

For example, previously, there could be implementations for maya and
nuke and when you installed the app for houdini things would just
install without errors, but when you tried to run the app it would not
find the desired built-in houdini hook because there was none.

Now instead, it will warn at install time and ask you to enter a manual
parameter value in this case.

/----------------------------------------------------------------------
| Item:    actions_hook
| Type:    hook
| Summary: Hook which contains all methods for action management.
\----------------------------------------------------------------------
WARNING: Sorry, no built-in support for the tk-shotgun engine yet! The
default hook value '{self}/{engine_name}_actions.py' for this hook
refers to an engine specific hook, however there is currently no hook
implementation available for the current engine. If you want to continue
with the install, you have to supply your own hooks.
Please enter value (enter to skip): 

Closes #81
For #25483 - changes to allow core to be zipped to improve performance
- Moved all resources outside of Python directory
- Fixed up various relative file path look-ups
- Added .../python/tank.zip to the Python path in the tank command
Added tank.zip for testing
renamed tank directory for testing
Renamed directory back to tank
Updated new dark palette code to work with modified resources structure
Updated tank.zip