explorer: do analysis and matching in a background thread

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?

Start by reading Lighthouse's referenced plugins/lighthouse/painting/ida_painter.py around line 70 to understand its threading approach. Trace capa's initial analysis and matching flow and the explorer plugin tab, then define completion as background work that does not block IDA, reports progress in the tab, and displays results there instead of in a popup dialog.

Written by the indexing model from the issue text.