Improvements for extended DSO data handling

[gzotti]

Is your feature request related to a problem? Please describe.

The extended DSO catalog is a separate download to be handled manually (see User Guide, 5.5.2). The file goes into the user data directory. The same name is used in each version, and it is not obvious which version is installed. An old file may conflict with a new version of Stellarium and vice versa. (?)

Describe the solution you'd like
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• make filename with versions. This would allow use with parallel installations of several (admittedly, only future) Stellarium versions. (Placing the file into the installation folder goes against our own recommendations.) Still, I think versions or other compatibility markers in filenames are simple to add. "Version 3.12" is very unintuitive, IMHO. Are all 3.* compatible, or what do major.minor numbers mean? (Am I blind, or where is that documented?) Why not just 0.20.4? Or 20.4 to make the next number strictly increase?)

• For tighter integration, add a download button in the settings dialog next to the star catalog download. The tooltip should include the warning about slow running.

Describe alternatives you've considered
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None so far.

Additional context
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[github-actions]

Thank you @gzotti for suggesting this enhancement.