Make the undo and redo stacks non-static members of AssFile, making it
theoretically possible to have multiple open AssFiles with working undo.
Slightly improve tracking of whether the file is modified: saving,
making a change, then undoing the change now results in the file being
shown as unmodified as with most programs with undo.
Add basic undo coalescing support.
Originally committed to SVN as r4667.
* Sort wx headers and wrap with #ifndef WX_PRE.
* Add missing heade rto stdwx.h.
M src/ass_attachment.h
M src/subs_grid.cpp
M src/ass_dialogue.cpp
M src/ass_attachment.cpp
M src/aegisub_endian.h
M src/audio_display.cpp
M src/audio_karaoke.cpp
M src/dialog_kara_timing_copy.cpp
M src/ass_dialogue.h
M src/text_file_writer.cpp
M src/stdwx.h
Originally committed to SVN as r3503.
