beautiful: Use weak references for tracking fonts

There is no point in keeping this font cache alive for longer than needed.
Letting lua garbage-collect fonts should provide sane cache-behavior in that we
forgot about the font once no one uses it anymore.

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/beautiful.lua.in

@@ -27,8 +27,8 @@ module("beautiful")
 
 -- Local data
 local theme
-local descs = {}
-local fonts = {}
+local descs = setmetatable({}, { __mode = 'k' })
+local fonts = setmetatable({}, { __mode = 'v' })
 local active_font
 
 local function load_font(name)