A cancel button is added to the `showProgress` dialog that is used only for rescans. When clicked, `AbortRescan` is called directly to cancel the rescan.

Rescans triggered from the debug console will now be cancelable by clicking the cancel button.

Rescans triggered by a command (e.g. `importmulti`) will now give an error indicating that the rescan was aborted by the user (either by the `abortrescan` command or by clicking cancel).

make check
./bitcoin-qt -> help -> debug -> console -> rescanblockchain -> select cancel in the new window.