Fix transparency plugin for display list/pref changes

There is no longer a buddy list/conversation split, so remove the duplicate settings (half of which were not used any more), and rename them without the differentiating prefix. Also, now that there's only one thing, there's no need for a second "enabled" setting in the plugin that just duplicates the plugin's enabled setting itself, so I removed it.

Instead of adding and removing a slider when the setting changes, I instead opted to always add it, and bind its visibility to the setting instead.

We no longer need a `gtk-config-frame` now that Pidgin uses GPlugin's plugin settings, so remove that and correctly wire up the settings schema so that GPlugin automatically displays settings.

Finally, the scale UI and alpha setting had different ranges. If the setting was moved below the scale's minimum, it would clamp it but not resend that value back to the setting, so things would appear inconsistent. To rectify this, I changed the setting's range up to match the scale.

Testing Done:
* Enabled Transparency plugin, changed around the transparency level and confirmed the main window became transparent.
* Toggled the solid-on-focus setting and confirmed the window lost transparency when focused.
* Toggled the slider setting and confirmed it was added/removed from the window.
* When slider was enabled, changed the setting and confirmed the slider changed and vice versa.

Reviewed at https://reviews.imfreedom.org/r/2394/