Feature-Plugin Install Directory

This is a subset of what I submitted with in the restructure feature. Currently, the plugin install directory is hardwired to being inside the includes directory. This feature will allow someone to enter put a __PLUGIN_TMP__ define in their include file to move this to anywhere on the disk. If no __PLUGIN_TMP__ exists, it will default to the old value.

This also adds a convenient button to install the files in the plugin.install directory.

If no one objects, I will delete the feature-Restructure branch and instead put it in a wiki.