Implement pango frontend without gtk+

[acertain]

I've been thinking about implementing a new backend, and here are my thoughts:

gtk+ is a rather large dependency, and it would be nice to not require gtk+, especially because it's tricky to get gtk+ to look nice on OSX.

After some thought, I think the best way to do cross-platform without gtk+ is just to use pango more directly, maybe on top of cairo.

There are 3 ways of doing this that I know of:

• pango+cairo+OpenGL: disadvantage: cairo has a gl backend that is disabled by default on most platforms, so we would probably need to ship our own cairo, advantage: shouldn't need any platform-specific code if we use say GLFW.
• pango+cairo's platform specific backends: advantage: cairo has these enabled by default, disadvantage: platform-specific code
• pango's platform specific backends: advantage: maybe this would be faster? disadvantage: more complex platform-specific code.

Note for all of these approaches, the haskell packages for pango and cairo don't include bindings for platform-specific APIs or the gl backend.

Shipping our won versions of cairo isn't IMHO such a bad thing, because on OSX using homebrew, the default version of pango and cairo (and gtk+) don't use Quartz, and instead use the ugly X11 system, so shipping our own versions could make yi much nicer fox OSX users by default. Also, installing external dependencies is a pain on Windows.

Finally, gtk+ gives us tabs, which could be implemented using cairo, but would be more work to do.

Note that GLFW would deal with input for us, and I don't know about dealing with input with the other options.

I'm planning to implement the first option.

[Fuuzetsu]

It seems like a big change for the sake of havaing Yi look nicer on OSX. Perhaps support on OSX should be fixed (how is it broken exactly?) and/or clear instructions provided how to make it look nice.

Will pango+cairo+OpenGL option end up being more work for us? You say that the advantage is being able to use GLFW but I don't know whether it is something that we want to do to begin with.

Using Qt as a cross-platform interface was also speculated but AFAIK there is little in way of Qt support in Haskell eco-system at the moment.