Proofread and updates for consistency of Swedish translation.
Fix crash from freeing backgroundsurface twice in MapRenderer

`backgroundsurface` is already freed in MapRenderer::clearLayers(), which
is called in MapRenderer's deconstructor.
Merge pull request #334 from hean01/master

Update of xgettext.py to support extraction of strings,
Merge pull request #332 from dorkster/bugfixes

Fix crash from freeing backgroundsurface twice in MapRenderer
Merge pull request #331 from hean01/translation

Proofread and updates for consistency of Swedish translation.