launch_external_viewer: Use full path when opening the file in a web browser

[seisman]

When opening a file via webbrowser.open_new_tab, we use the file URI with the format: file://{fname}. Here fname must be a full path, as shown below.

In [1]: import webbrowser

In [2]: webbrowser.open_new_tab("basemap.pdf")
Out[2]: True

In [3]: webbrowser.open_new_tab("file://basemap.pdf")
Out[3]: True

In [4]: gio: file://basemap.pdf: Operation not supported

This PR fixes the bug.