minor bug fixes:

initialize preview asynchronously to prevent GUI freeze
guard R calls with mutex, as R is not thread-safe
fix Drag'n'Drop
Merge branch 'master' of github.com:lfiaschi/simple-STORM

Conflicts:
	storm/wienerStorm.hxx
Only properly scale preview intensities if the zoom factor is >= 1,
else draw everything in white for incremental pixmap updates (no
need to iterate over entire reconstructed Hi-Res image)
Merge branch 'master' of github.com:lfiaschi/simple-STORM

Conflicts:
	storm/wienerStorm.hxx