Remove _M prefix

Textadept 7 does not encourage any prefix for custom, generic modules.
_CHARSET should no longer be necessary in Textadept 7
Fix bug when trying to open in another view
Fix indicator example
Rename core.gui module to core.ui
Update docs with new CSS and LDoc template
Fix docs after UI module rename
Fix documentation for key modifiers
Add note about events.INITIALIZED; unify spelling
Add example for `buffer._textredux` usage
Update changelog