Renamed `gui` to `ui` since it's more applicable.

--HG--
rename : core/gui.lua => core/ui.lua
Renamed `_M.textadept` to `textadept`; `_M` is for language modules only now.
Forgot to update some documentation with newly renamed `LEXER_LOADED` event.
Commented on the new role of `_M` in the migration guide; doc/14_Appendix.html
Fixed incorrect constants in buffer LuaDoc; core/.buffer.luadoc
Allow user scripts to handle `BUFFER_NEW` and `VIEW_NEW` events on startup.
A side effect is more efficient event emission during startup.
Renamed `buffer:get_style_name(n)` to `buffer.style_name[n]`.
Updated to the latest CDK.
Renamed `ui.docstatusbar_text` to `ui.bufstatusbar_text`.
Updated LuaDoc.
Fixed bug in processing command line options introduced in r1546.
Fixed potential crash caused by split views.
Updated for 7.0 beta 3.
Updated Lua Adeptsense.
Expose marker and indicator numbers for greater customization.
Removed their respective color settings.