Automate menu can freeze if checking for scripts takes too long

[petebankhead]

Bug report

Describe the bug
The Automate menu has got several submenus that are populated from directories that (may) contain scripts.

To ensure they are up-to-date, the submenus are populated whenever the Automate menu is opened. There's also a recursive check for scripts in subdirectories.

The problem is that, if it takes too long to check for scripts, then the menu never opens at all... making all the other options there inaccessible. Potentially (I think) it might also be possible to generate some infinite recursion via symbolic links.

To Reproduce
Steps to reproduce the behavior:

1. Go to Edit → Preferences...
2. Set the Script directory to be something particularly difficult (e.g. a network share, containing various subdirectories)
3. Try to open the Automate menu... and (at best) wait longer than you reasonably should

Expected behavior
The Automate menu opens, even if it's not possible to (quickly) find scripts to display.

Desktop (please complete the following information):

• OS: All
• QuPath Version: v0.3.2 (and earlier)

Additional context
Usually the 'fix' is to reset the preferences... which doesn't seem great.