Embedded dependencies workflow

[Guts]

Description

With this PR, I propose to close #166 with a development workflow allowing:

• integration with Dependabot (https://app.dependabot.com/) to track updates from embedded external libs
• update the 3 external libs to their latest versions
• later: remove the libs from this git repository, expecting from the developers of this plugin use the workflow (pip install)
• later: use the CI to automatically test against new versions
• later: use the CI to package the plugin with ugraded versions
• misc: connect to the dependency graphs like Github or libraries.io, displaying thanks and love to libs authors 🥰

The workflow is described in a specific doc page: https://github.com/QGIS-Contribution/QGIS-ResourceSharing/blob/42e481d86ef4d4b27f6592a12ad8d4468cbdbf71/docs/development/update_dependencies.md

Changes

• create a requirements file referencing the external packages
• document development workflow
• remove raw external libs from the repotisory
• move the ext_libs folder as a plugin subfolder

[havatv]

Looks great.Thanks!
It would be nice to ave an issue for this - could you create one, @Guts?

[Guts]

It would be nice to ave an issue for this - could you create one, @Guts?

Done. Tell me if that's ok.

remove raw external libs from the repository

I've added this step. I was thinking it's better to completely remove the raw external code from this repository. What do you think?

Additional question: is the gitter chan still in use?

[havatv]

It would be nice to ave an issue for this - could you create one, @Guts?

Done. Tell me if that's ok.

Looks perfect.Thanks.