Allow for Cate software updates without reinstallation

[forman]

Expected behavior

Software updates should not require reinstalling the software. Especially reinstalling cate-core requires installing a new Miniconda environment with is kind of heavy-weight. Ideally, an update is performed by using the CLI like

 cate update

Implementation: use conda CLI for package updates

For cate-desktop, there should be a menu entry Update in the Help menu.
Implementation: We can use Electron for incremental updates.

Important: when updating, both cate-desktop and cate-core must be updated!

Actual behavior

Cate software updates require reinstallation.

Specifications

0.9.0.dev7