Debug mode with user friendly test suite

[devmount]

Is your feature request related to a problem? Please describe.
After having #447 (which is already great), there is the need for a more user friendly way to see debugging results.

Describe the solution you'd like

• Implementing a section for testing stuff, either as part of the options page or as a dedicated testing page
• A button to start running tests
• Testing could run in the following steps (for now):
  • 1. Account processing: How many accounts can be queried and accessed
  • 2. Folder processing: How many folders can be queried and accessed, how many of which type (actual, virtuel, root, etc), which folders throw errors or take to long to process (e.g. more than 1min)
  • 3. Message processing: How many messages can be processed, which messages throw errors or take to long to process (e.g. more than 10s)
• The user should be able to see the progress at any time and see information or error messages which are actually useful (Like having the name/path of a folder that creates issues).

Additional context
Related to #445