Back to development mode.
Simplified end-user dependency to be just 'toga'.

Specific backends can be installed using toga[backend], but by default, installing just
toga will pull in the most appropriate backend for that platform.
Fixed #20 -- Ensure identifaction of all Linux platforms.
Improved implementation of set_platform().

These changes allow set_platform() to be invoked with no argument (to allow
'reset to system default', removes the need for an explicitly declared locals
variable, and allows any external user to manually specify their own backend.