Workaround for weird bug.

For unknown reason, some text files were getting an event in void
TextEntryPanel::onTextModified(wxStyledTextEvent& e) when opened,
despite the complete lack of any actual modification. Checking for
non-emptiness of the undo buffer prevents these ghost events from
mattering, but doesn't prevent them from happening.
MapTextureManager now keeps track of all existing textures/flats
Quick texture/flat selection in 3d mode now shows all resource textures (not just TEXTUREx/flats)
Merge branch 'master' of https://github.com/sirjuddington/SLADE
Remove FLOAT_ON_PARENT from parentless frame.

Fixes #131.
Merge pull request #132 from Gaerzi/master

Fix a couple of obscure bugs.
Merge branch 'master' of https://github.com/sirjuddington/SLADE into Music