PlayerSeekbar: thumb paint optimization

Painted thumb is now cached
PlayerSeekbar: code refactoring
PlayerSeekbar: allow frame numbers in the tooltip

For media that uses frame numbers for seeking, for example .swf
IMediaSeeking::GetTimeFormat refactoring
Options Dialog: properly gray-out inactive items

Refs #2449
MainFrm: don't hide window when changing tray icon

Fixes #3391