Update plugin download text in Preferences

Since there are now five plugins, instead of the three described in the
text.

I have changed the text to a generic "several plugins" so future updates
won't require re-translation.
Change beta version detection code to account for *.9

In this case, v5.9 is the beta for 6.0, not 5.10
Update project version to 6.0 beta

Because the next official PD release will break compatibility with old
file formats (macros, etc), I feel it appropriate to bump the major
version number accordingly.

6.0 (which would have been 5.6) will be a very large release with a host
of new features and a number of core design changes, further supporting
the decision to leave the 5.x line behind.

Now if only I had a release date on the horizon... ;)