PR: clear the googlemaps plugin cache on language change

[Dirk Hohndel]

> On Sep 2, 2017, at 5:33 AM, Lubomir I. Ivanov <neolit123 at gmail.com> wrote:
> 
> https://github.com/Subsurface-divelog/subsurface/pull/559
> 
> please, let me know in case someone thinks there are better ways to handle this.

I thought about creating language specific cache directories - but I just don't think
people are going to switch often enough to make it worth while, and actually, I think
it's more likely that people try another language and then either switch back or keep
the new language, in which case we'd habe one set of cached tiles sitting there
unused. So I think your solution is the correct one.

Pulled

/D