WIP: Aliases refactoring + closing #727, #716, #615, #704

Fix #727 (closed), #716 (closed), #615 (closed), #704 (closed)

A lot of changes ahead, but a nice bunch of issues fixed too 😄

The main changes are about aliases:

• removed AliasMixin class
• create_alias becomes a global function in alias module
• removed export_to_globals, remove_original options (set by default)
• always keep a strong ref to the underlying object in Alias
• removed 'hide_controller' option
• removed 'alias_or_fullname, alias_or_name properties
  • code explicitely ask for name, fullname or alias instead

Additional bug fix ?

• if there was a 'grouped_read_handler' the conversion function was not called... (see f6299)
• the test in test_sampling_counter_mode was checking for device.mode but device can be a GroupedReadHandler with no mode, I simply removed this check

Introduction of a new main module in bliss

• this is to avoid circular imports
• makes it more clear there is a main session
  • the main session is the session user started with, or last loaded one (not taking into accound sub-sessions of course)
• simplify imports and calling the main session
  • avoid session.get_current().

Settings

• introduced BaseSettings to factor out a lot of code
• same API for Struct and other settings, get rid of both _get_connection and _set_connection