map: PR for language support and new cache folder

[Lubomir I. Ivanov]

https://github.com/Subsurface-divelog/subsurface/pull/541

this PR contains a couple of patches:
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* mapwidgethelper: use dynamic creation of the Plugin object
this patch adds the PluginParameters support by making the creation of
the google maps Plugin object dynamic (e.g. via Qt.createQmlObject())
at the same time it adds language support for the map tiles.

language support for the tiles doesn't matter much for the Satellite
map type, but it matters for the Road and Hybrid map as those have
labels. by default it picks up the user system language, but that may
differ from the Subsurface UI language, so we use the helper
uiLanguage() to fetch it.

one side effect here is that if the user has already cached road map
tiles in one language, by switching the Subsurface language from
Settings, the plugin will start downloading tiles in another language
and it would end up with a mixture of tiles with different
languages....we might want to clear the tile cache if the Subsurface
language changes, but i would leave that to discussion,.

the plugin itself needs 2 small patch to enable support for a
PluginParameter for language and to enable Road map language support:
vladest/googlemaps#12

* mapwidgethelper: use system_default_directory() for cache
this moves the googlemaps tile cache to a the
system_default_directory() which is passed via a PluginParameter
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lubomir
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