QA: factorize GObject cast operations (datasource_file.c)

GObject cast operation consume CPU, so it could be useful to reduce the
number of call (even if it is low consumption).
Furthermore, it is better to have small source code lines.
QA: factorize GObject cast operations (datasource_gc.c)

GObject cast operation consume CPU, so it could be useful to reduce the
number of call (even if it is low consumption).
Furthermore, it is better to have small source code lines.
QA: factorize GObject cast operations (datasource_geotag.c)

GObject cast operation consume CPU, so it could be useful to reduce the
number of call (even if it is low consumption).
Furthermore, it is better to have small source code lines.
QA: factorize GObject cast operations (datasource_google.c)

GObject cast operation consume CPU, so it could be useful to reduce the
number of call (even if it is low consumption).
Furthermore, it is better to have small source code lines.
QA: factorize GObject cast operations (datasource_osm.c)

GObject cast operation consume CPU, so it could be useful to reduce the
number of call (even if it is low consumption).
Furthermore, it is better to have small source code lines.
QA: factorize GObject cast operations (datasource_osm_my_traces.c)

GObject cast operation consume CPU, so it could be useful to reduce the
number of call (even if it is low consumption).
Furthermore, it is better to have small source code lines.
Fix GLib macros typo
SF#3613971: Remove Geofabrik's OpenStreetMap WMS, as it is not for public use.

It no longer works without an appropriate account.