Create external plugin library

[GriffinRichards]

Loosely recording the changes needed to support a globally-available plugin library:

• (ce090b2) Rename all the "custom scripts" UI names to "plugins" (people sometimes confuse this name with map/event scripts)
• Create a new plugins directory in the global porymap config folder. Plugin scripts in this folder will be automatically accessible from any of the user's projects.
  • Plugins available this way should have a setting to say whether they are default enabled or disabled.
• Establish a metadata format for the plugin library (https://github.com/GriffinRichards/ppl). It should be possible to host plugin scripts either in that library repo or externally.
• Add a tab to the plugin editor that lists the plugins available in the library (as read from the library repo's metadata file). Each item in the list should have a title, a brief description, and a download button (or if already downloaded, an edit button and an indicator when an update is available)
  • Optionally, each item should also have access to author name(s) and a link to an external details page
• There should be some indication of the compatibility between the available plugins and the user's version of Porymap. Incompatible plugins should either be hidden or grayed out, unless already downloaded
• The new tab of the plugin editor should be disabled if the network module is not installed