ConversationWebView: Fix crash performing find in conversation

WebKit.FindController seems to be signalling found/not-found/cancelled
signals multiple times when typing text in the find entry (perhaps
when not found and cancelled due to the next keystroke?), causing the
async call to be resumed more than once and thus triggering a vala
assertion ensuring that doesn't happen.

This commit adds guards so that once the method has resumed once, it is
not resumed again.
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