Use a larger buffer to avoid a crash due to possibly long translations.
Fix two memleaks, thanks clang for noticing
Fix a clang warning about uninitialized memory

It was a false positive, but since we were using the global darktable.*
struct that couldn't be found out by clang. Now it's explicit.
Add option to filter images drawn on map

This reduces the images drawn on the map to the current collection/
filter, increasing the performance and reducing the mess on the map when
many images are geotagged within the current map section.
Also, changing map preferences doesn't require a restart any more.