Use default extension path if it's available

[petebankhead]

Feature request

Is your feature request related to a problem? Please describe.
QuPath currently requires the extension path to be set explicitly - even if the default path already exists. This can sometimes be annoying.

See https://forum.image.sc/t/default-extension-location-unset-by-default-problem-for-pre-installing-extensions/79594

Describe the solution you'd like

Proposed solution:

On startup, QuPath should check if there is an extension path set. If there is not, then it should check if a directory corresponding to the default path exists.

• If yes, ask the user whether to use this path
• If no, continue silently

Note that if the user does not want to use the path, there should be an option to select another path or to not ask again - otherwise the prompt will appear every time the software is started.

Describe alternatives you've considered

1. The default path could always be used if the directory exists, even if the path hasn't been specified.
   • This could be troublesome, because it means that extensions might be installed unintentionally. In the worst cases, if an extension is causing the problems that resulted in the QuPath reinstallation, then startup might fail.
2. There could be an option to set the extension path (including to the default) from the command line, which could be applied post-install.
   • This is already somewhat possible with a one-line script, see https://forum.image.sc/t/qupath-extension-folder-on-distant-server/78190/2?u=petebankhead

Additional context
An idea from the forum post was to set the extension path during installation, but I don't think there's a way to modify the installer sufficiently to get that to work.