Add Option to Restore POI on next Export

[wp99cp]

Add an option to restore the previous POI after restarting an export (e.g. after failed export).
Should be realised using the browser cache (currently, other fields are stored as well, e.g. map_scaling). However, POI should add a hash for the GPX file name (POI are bound to a specific route, whereas other settings as map_scaling, are threated as global settings and handled as consistent across exports).

Add a button clear POI, e.g.:

[wp99cp]

Update: It will be implemented once you can save a route (see also #358)