make window manager selectable from lumina-config>settings>general

[Nanolx]

There's not much to say. This makes the window manager selectable from lumina-config > settings > general.

Personally I'm not using fluxbox and used some modified session script in order to use Sawfish instead. This patch does not harm default configuration or code but gives some more power to the user.

Successfully tested switching between fluxbox and Sawfish a few times.

[beanpole135]

Hum...
I am fine with adding the support in the WMProcess class and using the session config file for setting it, but I am just not sure if we want to make it that easy to change to a different WM.
My reasons are this:

1. Fluxbox is the only supported WM by the Lumina project.
2. Changing to a different WM makes a lot of configuration options unusable (such as keyboard shortcuts, and workspace/theme configuration).
3. In my experience with trying different WM's for Lumina, I ran into lots of issues where using a different WM causes all sorts of strange breakage with general functionality of Lumina - specifically with regards to FreeDesktop standards compliance (Example: reserving screen space for the panels, and getting the list of open windows to show in the task manager).
4. By making it a simple option for the user to just select a different binary for the WM through lumina-config, it is quite easy to render Lumina completely unusable for people that are not familiar with the CLI.

Because of this, I am leaning more toward removing the WM selection option from lumina-config (unless it can be a lot clearer that changing it is not supported), and leaving it as an option for somebody to manually change in the config file (so only advanced users can change it).

Thoughts?

[Nanolx]

I'm fine with an hidden option aswell.

[Nanolx]

Done as said with above commit.

[beanpole135]

Looks good. Thanks!