In wxWidgets 3.15 (and perhaps older version) the call to GetTextExtent() probably
creates major issues when called too many times, at least oo msys2.
It was probably also time costly.
Calling it and resize the PROGRESS_REPORTER only when needed fixes these serious issues.
Issues happened only when the board has a lot of zones (when teardrops are added)
Also makes sure the progress dialog is closed when we're done reading
symbols (it used to stay up for much of the symbol editor initialization).
Also makes sure that any cancel in the preLoad step is honoured in the
sync step. (The preload is done because it is multi-threaded and therefore
faster than the single-threaded sync.)
Also makes sure that individual threads pay attention to the cancellation,
not just the GUI thread.
Fixes https://gitlab.com/kicad/code/kicad/issues/8372
jean-pierre charras
Jeff Young