[For 0.15.x] Reveal Plugins folder in the Unity Editor

[seandillon92]

Changed name of folder that containts the Plugin zip files from "Plugins~" to "Plugins". The "~" character at the end of a folder name, excludes the folder from importation in the Editor.

[seandillon92]

@sindharta-tanuwijaya I added a symlink from Plugins~ to Plugins. Tested with the Install Plugin button and it seems to be working well. Do you think we should keep the "Plugins~" folder in the paths mentioned in docs or replace it with "Plugins"?

[sindharta]

For this package, we can just replace it with Plugins

[seandillon92]

@schinkowski @SonnyCampbellUnity @mloccy another PR that a few others depend on. Would be good to review this one first