sdl2 icons workaround

[garlic0x1]

This fixes #1178

The issue exists on Arch and OpenBSD as far as I am currently aware, the freetype2 package that sdl2_ttf depends on is built without certain options on these systems that break icons in lem.

Using the icon registry, we go from having a pointer error, to just displaying an empty box icon, which is much preferred.

There are probably other icons elsewhere in the source that I haven't run into yet, but the solution is just to add them to src/base/icons.lisp

Before:

After:

[garlic0x1]

This change also allows users to change the icons in their config if issues arise, for example, the following will use '>' and 'v' in filer, and 'L' instead of the lock icon:

(register-icon "right-pointing-triangle" #x003E)
(register-icon "down-pointing-triangle" #x0076)
(register-icon "lock" #x004C)

[cxxxr]

Thank you.