obs-filters: Add a user label to the LUT filter

While discussing the Flatpak RFC [1], it was spotted that the
LUT filter couldn't open the file selection dialog. It was
explained, then, that the proper formats were either composed
of "User Label (file extensions)", or "file extensions", and
the LUT filter was setting "(file extensions)" without the
actual user label.

While this works on a standard Qt file selection dialog, it
cannot be properly formatted as a set of D-Bus filters, thus
breaking the sandbox integration.

Add a simple user label to the LUT file filter.

[1] https://github.com/obsproject/rfcs/pull/21#issuecomment-619106757

plugins/obs-filters/color-grade-filter.c

@@ -329,7 +329,7 @@ static obs_properties_t *color_grade_filter_properties(void *data)
 	obs_properties_t *props = obs_properties_create();
 	struct dstr filter_str = {0};
 
-	dstr_cat(&filter_str, "(*.cube *.png)");
+	dstr_cat(&filter_str, "PNG/Cube (*.cube *.png)");
 
 	if (s && s->file && *s->file) {
 		dstr_copy(&path, s->file);