GeoreferencingDialog: Hide 'Show reference point in' for local CRS

[dl3sdo]

When switching between local and geospatial CRS links to OpenStreetMap | World of O Maps were shown even if they were not clickable (for local CRS). This could also happen for a local CRS when using the Pick on Map and then pressing Reset. Besides that there was an orphan 'Show reference point in:' label for local CRS. The commit now uses setVisbible() for both, the label and the links, instead of just using setEnabled() on the links.

When setting up the dialog there was a doubled call edit_layout->addRow(show_refpoint_label, link_label); which lead to adding an empty row. This has been fixed by replacing it by adding a SpacerItem instead.

The last change corrects the link to OpenStreetMap. mlat and mlon coordinates are now created by 'f', 6 instead of 'g', 10 which is the appropriate format for defining the precision after the decimal point. BTW: 6 digits are equivalent to 11cm (or better).

[pkturner]

These changes look good to me.

[dl3sdo]

Rebase and split of previous commit to separate independent changes

[dl3sdo]

@dg0yt, as you already reviewed the first three commits, can you please review the last one (ad0378d) ?