[Qt] Fixed memory leak: Transaction Dialog
[Qt] Fixed memory leak: Transaction Dialog

Transaction Dialog was not getting gc'd and it would live in memory
until the wallet window was closed.  This have been fixed now.

- Had to add a couple of weak references for some of the lambdas it uses
- Had to make sure that it cleans itself up properly on close
(disconnects signals and removes itself from any main_window lists it
may have been added to).

Feels good to save memory. :)

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