[syntax_highlight] Integrate with master branch

This commit contains all changes starting with the first integration of
the plugin into the gajim_1.1 branch (76dabe2) until the current plugin
version V3 (42b9aeb).
The manifest.ini is updated for compatibility with upcoming Gajim
versions.

syntax_highlight/types.py

@@ -0,0 +1,21 @@
+from enum import Enum, IntEnum, unique
+
+PLUGIN_INTERNAL_NONE_LEXER_ID='_syntax_highlight_internal_none_type'
+
+class MatchType(Enum):
+    INLINE      = 0
+    MULTILINE   = 1
+    TEXT        = 2
+
+@unique
+class LineBreakOptions(IntEnum):
+    NEVER       = 0
+    ALWAYS      = 1
+    MULTILINE   = 2
+
+@unique
+class CodeMarkerOptions(IntEnum):
+    AS_COMMENT  = 0
+    HIDE        = 1
+
+