add radiobutton
fix basic functionality of radiobutton
fixing radiobutton widget, adding it to local.mk
add a not-working hashmap

Hashmap will be used on radio buttons to ensure that only one radio
button is selected among the ones with the same name. Every entry in the
hashmap will be a list of radio buttons with the same name.
add insert operation to hashmap

By the way, the typedefs in base_type.h are changed. They now use types
declared in stdint.h as base types.
add grow, shrink and erase operations to hashmap
hiding implementation in hashmap.c

Also adding hashmap into compile system, fixing compile errors in
hashmap and radiobutton.
finish writing radiobutton with hashmap

Not tested yet.

Also made a minor fix in list.{c,h}, making a function declare to return
the correct type (struct list_node *).
radiobutton almost useable

Several fixes about client crashing. Added a test suite in
multi_windows.c.
Merge branch 'radiobutton'
minor fix about user-defined callback in checkbox

Forgot to consider the case where the user don't provide with a callback
function. Client programs would crash on this.