Improvement: Add tinting to course-index activity icons

[pkgh]

I have been playing with coloured / tinted activity icons the course-index over the last few months of this academic year (under Moodle 4.5, then Moodle 5.0), and am finding them a much pleasant look than the default black icons. A sample:

Thus, I would propose (for Moodle 5.1) that once the "Display activity type icons in course index" (theme_boost_union | courseindexmodiconenabled) setting is activated, the user could be given the option to also apply tinting / colouring of the icons as compliant with the settings of "Activity branding" (admin/settings.php?section=theme_boost_union_look#theme_boost_union_look_activitybranding)

(However, proprietary icons should be left alone as they are IMO - cf. #953 (comment) .)