Addressed some issues from comments

Renamed the variable 'pluginjson' to 'plugins_available'

Removed checkboxes from the plugins download list.

Renamed the file 'plugins_dl.py' to 'plugins_download.py'
Installed plugins are not shown in the available list

This keeps things clear - two lists for distinct purposes - Installed &
Available.

The "Hide installed plugins" checkbox has been removed and the groupboxes
renamed.