Close PlugInDialogs in IJ1Helper.dispose()

[gselzer]

If we just call PlugInDialog.dispose(), that does not perform the shut-down needed to e.g. the infinite run loop of PlugInDialogs like ThresholdAdjuster.

I did a little more than that though - it seems like getAllNonImageWindows could allow us to remove a couple of helper methods!

[hinerm]

@gselzer this works for me in resolving the hanging issue in https://github.com/imagej/napari-imagej/issues/141

@ctrueden do you see any immediate problems from an ImageJ 1 API perspective with this change?

If not I think we should merge.

[ctrueden]

Thanks! Awesome improvement!