Change how platform support is defined

[fohrloop]

Closes: #377

Summary

• Replace PlatformName with PlatformType and IdentifiedPlatformType
• If platform is unknown, do not fail in the platform check
  phase anymore, but continue trying using the method.

Before

PlatformName constant, which is used to both: (1) As type for current
platform (2) As types to list in subclasses of Method in the
supported_platforms attribute.

The problem was that the supported platforms definition was not very
flexible. For example when adding support for FreeBSD (GhostBSD), in
#359, one would have needed
to add the PlatformName.FREEBSD to each of the methods, like this:

class SomeMethod(Method):
    supported_platforms = (PlatformName.LINUX, PlatformName.FREEBSD) 

Now

Now there's the possibility to use

class SomeMethod(Method):
    supported_platforms = (PlatformType.UNIX_LIKE_FOSS, ) 

where the UNIX_LIKE_FOSS contains all FOSS Unix-like systems (linux+bsd,
but not MacOS or Android). Adding now systems to the list does not
require any changes in the Method subclasses supporting UNIX_LIKE_FOSS.

There are now two types of constants: (1) IdentifiedPlatformType, which
is used as the type of the current platform and (2) PlatformType, which
is used in the subclasses of Method in the supported_platforms
attribute.

New PlatformTypes

PlatformType.BSD: Any BSD system. Currently only consists of FreeBSD but
the list may grow in the future.

PlatformType.UNIX_LIKE_FOSS: Unix-like desktop environment, but FOSS.
Includes: Linux and BSD. Excludes: Android (mobile), MacOS (non-FOSS),
ChromeOS (non-FOSS)

PlatformType.ANY: Any platform.

New behavior

Platform support check does not fail (but returns "I don't know"/None)
if the platform could not be detected. This means that methods like
org.gnome.SessionManager will be tried also on non-linux systems (which
might be using Gnome).

BACKWARDS INCOMPATIBLE CHANGES

The name of the enum class to use in Method.supported_platforms was
changed from PlatformName to PlatformType. This only breaks code which
uses custom wakepy.Methods, and the fix is to rename the variable.

Other small changes

get_platform_supported() now takes the current platform and supported
platforms as arguments (previously, used Method and current platform).