Stop distinguishing between up/low-case file ext for custom icons

[maicher]

For custom icons (set in the LF_ICONS env), does it make sense to distinguish the lowercase from the uppercase file extensions? I mean if someone sets the icon eg. for *.png files, it would be nice to show it also for the *.PNG.

Before:

After the change from the PR:

[rafcon-dev]

Please merge this

[lahwaacz]

This was implemented in #908 (merged), so this should be closed.