This modifies the Spellchecker abstraction to add a new dictionary support, with
support for pyspellchecker. It also changes the main UI so that multiple libraries
can be supported and dictionaries provided to the user. The custom dictionary of
pyspellchecker has to be handled manually, and the performance and words of this
library isn't on par with PyEnchant, but at least it works with 64 bits.
Fixes#505
This is in preparation for adding support for additional spellchecking libraries
other than PyEnchant which seems to be unmaintained and does not build in
Windows 64 bit.