Improve the TODO

I've had a few comments that I thought would be good to add to the TODO.

Also, this starts drafting a roadmap for what we need to have before
releasing the first stable release, and what could be potential
evolutions in the future.
Add one more item to the TODO list
And two more items to the TODO list
Properly implement the display of radicals in the input box

Now that libcangjie added `CangJie::translateInputKeyToCangJie`, we can
just use that instead of our hacked-up stuff.
Remove useless stuff

No idea why this is here, probably some copy-paste gone wrong
while I was still trying to understand the Autotools...
Move the setup tool to bindir

This is a user program, as such it really should be installed in her
PATH, so she can execute it.
Move the engine script to pkglibexecdir

First, this sets libexecdir as $(prefix)/lib, because really, that's
where this kind of things should be.

However, to avoid polluting e.g /usr/lib, we now install in our own
sub-directory.
Remove obsolete text

We moved to only support Python 3 a while ago.
Don't check for ibus in configure

We shouldn't need it at build time.
Properly document the dependencies in the README

This should greatly help testers and distro packagers.
Remove the 'jhbuild' folder

This was a nice try, but I never managed to actually make it work.
Remove the mention of libcangjie from the dep list

We don't directly require libcangjie, we require pycangjie (which itself
requires libcangjie).
Remove done TODO

I just went over the 3 projects (libcangjie, pycangjie and
ibus-cangjie) and I think we're now checking for everything properly.
Remove a done item from the TODO

I've added the note for libcangjie and pycangjie, it's unnecessary for
ibus-cangjie.
Remove an item from the TODO

I moved it to a bug report:
    https://github.com/bochecha/ibus-cangjie/issues/1
Remove future stuff from the TODO

I moved them to bug reports, in their own special milestone:
    https://github.com/bochecha/ibus-cangjie/issues?milestone=2
Remove the TODO file

Everything which remained has now been moved to bug reports, in the 1.0
milestone:
    https://github.com/bochecha/ibus-cangjie/issues?milestone=1
cangjie: Pressing space twice on multiple candidates

When pressing space, the user sometimes gets multiple candidates.

When that happens, if the user presses space again, we should commit the
first candidate.

fixes #7
Simplify the settings

There are only two choices possible for the full/half-width characters,
so a combobox is a bit overkill.

Also, it's not about punctuation only, but also numbers, letters,
symbols,...

We have a good default, so we might as well use a simple checkbox.
Reorganize some code

This is just to make the next commit easier to review.