Move trends.earth settings into main QGIS settings window

[azvoleff]

Currently settings for Trends.Earth are accessed from the plugin toolbar:

This brings up what has become a very cramped window:

We should move these settings into the main QGIS options screen:

We do need to keep the functionality to change regions and the land cover legend easily accessible though, so we'll need a way for a user to click on the "Change region" button in the various tools (and similarly for land cover legend) and directly bring up that page in the main QGIS options window.

[azvoleff]

Also see #397

[vermeulendivan]

This has been done: https://github.com/ConservationInternational/trends.earth/pull/705