Replace the EVO_EXPRESS environment variable with an --express command
line option.  (Note, this adds a new translatable string for --help.)

Add an EUIManager class with an "express-mode" property and custom load
functions that use our new "express" preprocessor.  This replaces the UI
manager functions in e-utils.c.

(Also going to see if I can get GTK+ to add an "add_ui_from_string"
method to GtkUIManagerClass that we can override.  Then we could just
call gtk_ui_manager_add_ui_from_string() and the preprocessor would
automatically do its thing and chain up.)

Add an "express-mode" read-only GObject property to EShell.

Add e_shell_configure_ui_manager() to e-shell-utils.c.  For now this
just creates a one-way property binding:

    EShell:express-mode -> EUIManager:express-mode

Call this immediately after e_ui_manager_new().  (EUIManager can't do
this itself because it lives too low in the dependency hierarchy and
doesn't know about EShell.)