make projection setting transient

[bastiK]

It happens that users change the projection once (e.g. to WGS84), then forget about it and are unaware they are using a projection that is unsuitable for mapping (e.g. #14799). This is much easier, now that background imagery is silently reprojected.

I suggest to save the projection only for the duration of the current session and reset the projection to Mercator when JOSM is restarted. In addition I would add a button "Set as default" for advanced users to make the projection change permanent.

An added advantage would be, that we can safe the projection settings in the .jos session file. Then it would be natural that projection is changed when session is opened (and possibly reset when closed).

[stoecker]

If you drop prefs you should add them to the cleanup list, so they are removed.

[bastiK]

In 12306/josm:

fixed #14877 - make projection setting transient

[bastiK]

In 12485/josm:

remember projection choice sub-preferences (e.g. last selected UTM zone) across sessions (not just when changing the default projection) (closes #15013, see #14877)

[bastiK]

In 12486/josm:

see #15013, see #14877 - remember projection when session is saved to a .jos/.joz file

[bastiK]

In 12487/josm:

add null checks (see #14877)

[Don-vip]

In 12488/josm:

see #14877 - checkstyle