Improve filter on import

Instead of showing all images, we set the filter to the smaller of the
default import rating and the current filter. This ensures that the
imported images are shown, but that we don't reduce the filter
unnecessarily.
Update comments for improved import filter handling
Keep old import filter reset for collect operations
A few new strings
lighttable: enable full-res thumbnail when pressing `z'.

unfinished, needs better user interface/option in prefs for it.
preview: fix memleak and some rounding errors
preview: fall back to old `z' in case there is no thumbnail in the file.
Set description in Picasa export only if it is defined

Otherwise Picasa displays filenames as photo descriptions, which is not nice in exported albums
Use Google's terminology in variable names - title/summary, instead of caption/description

Title is the filename and summary is something that is displayed along with the photo on the web (xmp title).
Merge remote-tracking branch 'origin/master' into full-res-preview
preview: don't jump when mouse exits drawing area.
preview: take orientation into account.
preview: initial sharpness detection code.
focus: initial focus cluster regions implementation.

needs weighted line width/color to indicate sharpness
focus: some attempt to rate and visualise sharpness regions
focus: try two-level cdf22 wavelett.

so far a lot worse than the simple version.
focus: some tweaks to sharpness detection.
Merge remote-tracking branch 'origin/master' into full-res-preview

Conflicts:
	src/views/lighttable.c
focus: make clusters detect more robustly, and draw over regular thumb.

red: spot on focus
blue: one wavelet level later (for high iso shots)
lua : fix a bug in documentation generation