explorer: do analysis and matching in a background thread

[williballenthin]

avoid blocking the IDA UI while doing the initial analysis and matching. show a progress bar in the plugin tab rather than the popup dialog. once the analysis is complete, show the results in the tab.

lighthouse seems to be using threads to enable high performance painting of the database, and has details here: https://github.com/gaasedelen/lighthouse/blob/develop/plugins/lighthouse/painting/ida_painter.py#L70

notably, we only read from the database, not write, so should be even easier to do in the background?