cleanup exif data

[zisoft]

See discussion in #16479

It turned out that exiv2 gives a whole zoo of manufacturer specific values for the white balance. In some cases where a camera value has no representation by exiv2, the color temperature value is returned, in a not well formated way with trailing spaces.

Improvements/fixes:

• remove trailing spaces
• not defined and unknown are now treated as the darktable default unnamed
• entries are now compared case-insensitive before they get written to the database. This avoids having duplicates like Cloudy and CLOUDY
• if a numeric exif value n has no textual representation in exiv2 the returned string is (n) which is pretty much useless so it is discarded now
• fixed a bug in collection.c where strlen() was used instead of g_utf8_strlen(). This failed on strings containing special characters (e.g. german umlauts)

[TurboGit]

Some minor style issues to fix.

One comment, in the description you have:

not defined and unknown are now treated as the darktable default unnamed

Where is this happening? I cannot see this in the code...

[zisoft]

Where is this happening? I cannot see this in the code...

Not defined and unknown are the texts coming from exiv2 if the exif value is 0.
Snippet from the exiv2 source:

//! ExposureProgram, tag 0x8822
constexpr TagDetails exifExposureProgram[] = {
    {0, N_("Not defined")},       {1, N_("Manual")},           {2, N_("Auto")},
    {3, N_("Aperture priority")}, {4, N_("Shutter priority")}, {5, N_("Creative program")},
    {6, N_("Action program")},    {7, N_("Portrait mode")},    {8, N_("Landscape mode")},
};

//! MeteringMode, tag 0x9207
constexpr TagDetails exifMeteringMode[] = {
    {0, N_("Unknown")}, {1, N_("Average")},    {2, N_("Center weighted average")},
    {3, N_("Spot")},    {4, N_("Multi-spot")}, {5, N_("Multi-segment")},
    {6, N_("Partial")}, {255, N_("Other")},    {255, N_("Other")}  // To silence compiler warning
};

So I simply avoid setting the text by checking the exif value. In case of 0 no text is written which is later in the collection filter module handled as unnamed

[TurboGit]

Lot better now, the collection entries are clean. Thanks a lot!