medialibrary: Correctly represent imported vs total clips in the progressbar
medialibrary.ui: Use a smaller, symbolic label for warning_infobar close button

The use of the "Close" label made the button a bit too prominent, especially
in languages such as French. Besides, one can argue that the verb "close"
may be weird in the context of an infobar inside another widget.
Using the "✖" unicode symbol and a tooltip instead.
medialibrary: Show the amount of errors with ngettext in warning_infobar

Until now we just said "Errors occurred while importing", so we might as well
say the exact number to let the user know how many clips failed to import.

This is also demonstrates Python's new standard way of formatting strings.
Update French translation and improve the French AppData.xml description
Updated Polish translation
Updated Slovenian translation
Updated Czech translation
Remove unused imports and fields
medialibrary: Reuse the code for setting the dragged paths
medialibrary: Fix supported file formats lists
medialibrary: Use a local variable instead of a field for the error dialog box
medialibrary: Pythonic check
medialibrary: Allow importing MTS files
medialibrary: Pythonic iteration over supported file formats
medialibrary: Pythonic checks
medialibrary: Ignore all the uris which evaluate to False
medialibrary: Clarify the drag and drop handler
previewers: Do not fail if thumbnail timestamp is not the requested one

We should not miserably fail if the video thumbnail's timestamp is
not the exact requested one. Instead we try to figure out what the
timestamp means.
previewers: Optimize thumbnails reuse
previewers: Pythonic thumbnail time iteration