Prefer compiler warnings to assertions for unhandled switch cases

Prefer compiler warnings to assertions for unhandled switch cases

Using 'default:' cases can hide logical errors which would lead to i3
crashes for users. With this change the compiler will print a warning
when a case is not handled. For example, if I add a new value in the
Font.type enum:
../../i3/libi3/font.c: In function ‘draw_text’:
../../i3/libi3/font.c:378:5: warning: enumeration value ‘NEWFONT’ not handled in switch [-Wswitch]
     switch (savedFont->type) {
     ^~~~~~