Global: add categories

[leograba]

The concept of categories is to tag similar entries together. There are separate categories for each different type of entry: expeses, revenues, loans, investments.

There must also be mandatory categories, which are not editable nor deletable, and optional categories which are the opposite.

The categories themselves are TBD.

• Find out how many types of entries there are.
• Create the logic and DB structure for category handling, such as "add", "edit", "delete"
  • Create a unique ID for each category and keep this ID when editing. Always link by ID and not by name, so categories edition does not mess with the list of entries.
• Define which categories are mandatory. Provide at least one category for each type of entry (e.g. expenses).
• Provide a set of optional categories that are default to a new login. The user is able to delete or edit them.