#1827 GTK: Fix crash on widget destruction

[Colomban Wendling]

Avoid possible IdleWork() calls on destructed ScintillaGTK objects.

The idle callback removal clause in the ScintillaGTK destructor got broken in [code:4e5c321dda8e69104cb7b639c3107dc11dbc00f6], leading to the callback not being removed thus possibly running after the instance destruction.
Indeed, gdk_threads_add_idle() wraps g_idle_add() with a custom user data, thus making the g_source_remove_by_user_data() call in the destructor incorrect, as we give it our own user data, not GDK's wrapper one.

Fix the callback removal not to use user data matching to avoid this.

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Attached are 3 possible implementations of a fix:

• v1 transforms WorkNeeded::active to an IdlerID so the GTK platform can use it as such. No other platform seem to use WorkNeeded::active.
• v2 removes WorkNeeded::active and adds a private member to ScintillaGTK with the same purpose. Again, no other platform seem to use WorkNeeded::active, hence the removal.
• v3 is v2 but without the WorkNeeded::active removal.

[Neil Hodgson]

The other core platform that uses a high-priority idle for styling is Cocoa. Adding styleIdleID is more similar to Cocoa and uses GTK+ types avoiding casts so this was the alternative I preferred. Removing active could potentially break other platforms but it wasn't used by Cocoa so appears fairly safe.

Committed alternative 2 as [d88920].