Add more typical main block

This is simply a convention which allows you to be freer to import the
FaceBase_Launch.py file into another script without it launching the UI.

Only when the FaceBase_Launch.py script is executed directly will the
built-in __name__ variable be defined as "__main__". During import it
will be the name of the file itself and so won't trigger this
if-statement.
Improve file menu tool tips

Correcting the previous copy and paste content to something suitable but
basic.
Improve tool tips for selection filter buttons

Rephrased in a more tool tip style.
Add simple status bar message logger

We create a message logger class which wraps the status bar from the
main application window and exposes a simple message logging interface
to the rest of the code.

We pass it to the RigGraphicsView and use it to report one of the errors
in a simple string manner. Should be improved to use a coloured error
panel. Most of the change in RigUIControls is a space change due to
reindenting the function after moving the error check and exit to the
top.
Centralise some of the stylesheet reading

We switch to reading the stylesheet information up front and complaining
if it isn't there. We then pass this into the classes which care about
it rather than reading it in the instance set up code.

We also move to having a main() function just to make it easier to exit
out of the set up code if needed. We also return an appropriate value on
error or successful exit. This isn't really important for a GUI
application but is good practice.
Change message logger to use styled red QLabel for error

This is also a more permanent message than the more temporary
QLabel.showMessage method. It is hidden by other temporary messages like
status bar tool tips but comes back until we hide it ourselves.

We don't have a mechanism for hiding it yet, we should investigate
sending a "clear" signal or something after successful user actions.
Added very simple clear method to StatusBarMessageLogger

Add called it from the bottom of one of the more common actions so that
if some action sets an error, it should get cleared on the next
successful action. This needs more work but would benefit from some
larger structural changes first.
Merge branch 'status-line'