use read/writeSessionConfig to load/save session config data, i.e.:
* last 10 search patterns
* last 10 paths
* last 10 file filters
* recursive and case senstitive

This is the first plugin which uses read/writeSessionConfig, and - of course -
it does not work: Right now, the mainwindow-number is always -1 in readSessionConfig, however, writeSessionConfig seems to work.