Installation changes:

Installation changes:
	* Added installer.update() which Updates the installed copy of NVDA to the current version being run.
	* gui: Added an UpdaterDialog which contains an update button, which calls installer.update to update the currently installed NVDA. The "install NVDA..." item in the tools menu now will show the update dialog rather than the install dialog, if a previous copy of NVDA is installed.
	* installer.py: Added a few useful functions such as getStartMenuFolder, getInstallPath, validateStartMenuFolder and validateInstallPath. These are now used by the installer GUI to collect default/current installer values.
	* installer.py: install only now calls unregisterInstallation if forUpdate is true. Otherwize we expect there is no previous install and or the previous install has already been uninstalled.
	* installer.unregisterInstallation: don't remove individual startMenu shortcuts, instead just delete all shortcuts in NVDA's startMenu folder, as this is wht the old uninstaller used to do.
	* installer.registerInstallation: fix a typo that was stopping start menu shortcuts from being created.