Preferences Dialog does not update after extension is enabled/disabled

[zerwas]

After enabling or disabling an extension without closing the preferences dialog, the options available are not automatically updated, which may cause freezes if an option that should no longer be available is toggled.

For example:
1. Enable the BPM detection extension
2. Switch to the "Source Specific" tab
3. Notice that there is no "Automatically detect BPM for all songs" option.
4. Close preferences dialog.
5. Reopen preferences dialog.
6. Repeat step 2. Notice that the option mentioned in step 3 is now present.

Another example:
1. Disable the BPM detection extension
2. Switch to the "Source Specific" tab
3. Notice that the "Automatically detect BPM for all songs" option is still present.
4. Toggle option
5. Notice that Banshee hangs.