Cleans up some duplication for each page.

Reviewed-by: Friedemann Kleint

Start the jobrunner on demand, save options only if changed,
give it the structure used elsewhere in the VCS plugins. Remove
last traces of the cmd /c hack.

Do not use "cmd /c hg" on Windows as this will fail randomly
depending on arguments with blanks, such as "-U 8". Output log
messages about synchronous commands, format author correctly,
label menus correctly.

Polish: Canconical slots, const-correctness, some QLatin1Strings/tr,