Fix deadlock for plugins that create widgets

[juancampa]

Problem

Originally reported by @emilk in #7652 (comment)

When a plugin creates a widget (e.g. during on_pass_end) and that widget gets hovered, egui will call on_widget_under_cursor, causing a deadlock because the plugin is still locked on the stack.

Fix

Added Mutex::try_lock which returns None immediately if the mutex can't be acquired.

This is only used for on_widget_under_cursor, which will now get skipped if its mutex is locked.

Since plugins can technically be locked from other threads (via TypedPluginHandle), the try_lock function is not used for the other plugin hooks so that they execute reliably (as they used to before this change). Whereas on_widget_under_cursor will be skipped if it happens that the plugin is locked externally. Small caveat but probably not an issue?

For example, a plugin does some background work and uses TypedPluginHandle to set the result back in the plugin state object. To set the result, it locks the plugin. If it happens that egui tries to run on_widget_under_cursor, it will be skipped.

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[github-actions]

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