Add drag-and-drop plugin validation to the main window

[jcredland]

Summary

• Drop a .vst3 / .vst / .component / .dll / .so / .clap anywhere on the pluginval window to validate it.
• Shows a translucent rounded-rectangle overlay with a "Drop plug-in to validate" label while dragging an accepted file.
• Routes through Validator::validate(StringArray, options) — the same flow used by Test File…, so it picks up the current strictness level, timeout, sample rates, etc. Console tab is auto-activated when validation starts (existing behaviour).
• Mixed drops accept the plugin files and ignore the rest; drops with no plugin files are a no-op.

Test plan

• Drag a .vst3 onto the window — highlight appears, drop starts validation.
• Drag a non-plugin file (e.g. .txt) — no highlight, drop is ignored.
• Drag multiple plugins at once — all are queued via a single validate call.
• Drag onto the Console tab as well as the Plugin List tab — both trigger validation (plugin list table doesn't intercept drops).

[sudara]

Thanks for this!

I QA'd this and looked through it.

The prior behavior is that a drag would add a plugin to the list (vs. run validation). It was the main way I added plugins (dropping them in from finder). I do think that still makes sense to add, even though it's also useful that it starts a run. We could do both with a change like this: 6ec3eb6

The other thing is that with the run-on-drop behavior, I can reliably beachball pluginval by dropping a plugin in there while another is scanning. Something like this would fix: c7b155a

Otherwise looks and works great, happy to merge with my commits as followups or wait for changes.

[drowaudio]

I guess ideally you'd want the option to validate or add to the plugin list (scan)?

[drowaudio]

How about when you drag over, the two overlays appear, "Validate" and "And to list" on the left and right halves. Then you drop on the left or right for the desired behaviour?

As you drag left/right, that section would need to highlight to obviously show the action to be performed.