Add context menu to utiLITI resources

[steffen-wilke]

Is your feature request related to a problem? Please describe.
Managing resources in utiLITI could be improved by supporting a context menu for the resources tab that allows to access already present functionality.

Describe the solution you'd like
Functions that should be part of the context menu:

• All resource import functions from the main navigation (e.g. "Resources -> Import -> Spritefile...")
  This should be context sensitive to the currently selected type of resource
• Export functions for any type of resource (e.g. "Export Spritesheet")
• All resource actions that are accessible when a resource has the focus
  • "Add to environment"
  • e.g. "Edit Spritesheet"
  • e.g. "Delete Spritesheet"

Additional thoughts
Also, keyboard shortcuts would be a good way to access this functionality from the resource tab