Made some methods const

There was never any reason for these not to be const methods, so I made
them so.
Constructor cleanup

Made the constructor better
Minor bug fix

mTextForms would always have at least one TextForm in it at this point,
so the comparison was meaningless. (Also changed a length() to count()
for consistency.)
Added ability to change the baseline text

This has been possible for a while, given the data structures, but now
there is an interface.
Bug fix

Making sure that new GlossItems are properly connected to the phrase.
Bug fix in deleting Text objects

Just careless code. It's important to delete Text objects, or the
concordance will eventually cause a crash.
Removed redundant function calls

These calls were reached anyway through calling appendGlossItem
Patch for last

Another redundant function call.
Replaced C-style pointer casting

qobject_cast doesn't work in this case because GlossItem has already
been destroyed by the time this slot is called. It has to be C-style
pointer casting, or else rewriting the containers to take QObject
objects (yuck).
Proper TextBit comparsion

This should compare the complete TextBit objects, so that writing system
isn't neglected.