refactor(plugins): fix plugin loading data flow

[imsnif]

This changes the way plugins are loaded on startup and when opening a new tab with a layout.

Up until now the plugin loading blocked the Screen thread (that loaded them) for each plugin. After this change, the plugins are loaded separately in the plugin thread before the reach the screen thread.

The new flow:

1. server router
2. plugin thread => loads the plugins and forwards their IDs
3. pty thread => loads the terminals and forwards their IDs
4. screen thread => populates the layouts with the above IDs and renders the screen