move pluginhost into subdirectory

so it can be build separately
ignore some VS files
pluginhost source files have moved
pluginhost headerfiles have moved
moved old/ to legacy/

to make it more clear that this is still of interest
vsprojects: renamed "old" to "legacy"
removed headerfiles filter
define to enable legacy-builds
moved headers into include directory
include files in include/ directory
udpated makefiles for old->legacy rename
only include plugin.h when in non-legacy mode
build legacy host