Replace "languages" by "selector"

[rwols]

• Maintain a map of base scopes -> language IDs
• Rename the awkward feature_selector to priority_selector
• Solve the problem of non-unique language IDs in SessionView
• Update all default clients
• "languages", "document_selector", "syntaxes" are all backwards-compatible
• But not if you use LanguageConfig in your plugin somewhere

This is a minor version update because the public API changed.

Resolves #1384

[rchl]

Will try to look at it later today.

[rchl]

My not-very-clean ST shows one test fail:

======================================================================
FAIL: test_can_parse_client_settings_with_languages (test_configs.ConfigParsingTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/rchlodnicki/workspace/lsp/Sublime LSP.app/Contents/MacOS/Data/Packages/UnitTesting/unittesting/core/py33/case.py", line 26, in _executeTestPart
    deferred = function()
  File "/Users/rchlodnicki/workspace/lsp/Sublime LSP.app/Contents/MacOS/Data/Packages/LSP/tests/test_configs.py", line 28, in test_can_parse_client_settings_with_languages
    self.assertEqual(config.selector, "source.python")
AssertionError: '' != 'source.python'
+ source.python

[rchl]

This is a bit over my head right now but it seems to work so I approve. :)