Add to map button for orders

[vermeulendivan]

This mostly focusses on a new feature which allows the user to add a downloaded layer to the QGIS canvas. Other changes includes the following:

• Buttons has been moved to be at the bottom of each element in the orders list, as apposed to on the right;
• Buttons are next to each other, and no longer stacked on each other;
• The layer will be added as BGR (true colour) and not RGB; and
• Error checking. For instance, if the manifest.json file is missing, or if the image it tries to load is missing, the user will be presented with an error message. This can happen if the downloading unexpectedly stopped, or if the user deleted/moved the downloaded data.

[vermeulendivan]

Accidently merged this, meant to close it. The new PR for this has been done from a branch from within the main repo: https://github.com/planetlabs/qgis-planet-plugin/pull/77