BUILD: Rename libunity support variable to "USE_UNITY" instead of "USE_TASKBAR_UNITY".

This makes it consistent with other library support variables.
BUILD: Auto detect taskbar support.

Formerly the taskbar support was *always* enabled except when the backend or
user specified that it should be disabled. This causes nasty crashes for
backends which do not have any taskbar support (like DC, Tizen and probably
more which simply did not disable it so far) when defaultErrorHandler was
called for example (Mass Add is also broken for those).
The SDL (and derived backends) worked around missing taskbar support by
simply faking a dummy taskbar implementation (but still claiming in configure
that we feature taskbar integration, ouch).

To avoid all non-SDL backends from manually specifying _taskbar=no I added
some auto detection code which simply only enables taskbar support in case
ScummVM is built on Win32 or libunity is present.
Merge pull request #421 from lordhoto/taskbar-cleanup

BUILD: Taskbar Cleanup/Fixes