Introduce UDP custom data source

[Kittyfisto]

(See as to how this feature got to be: https://github.com/Kittyfisto/Tailviewer/discussions/253)

Users should be able to add custom data sources which aren't based on concrete files but can rather be accessed via some API.
This issue shall include:

• A concept which defines how users add, configure and access new custom data sources
• A new plugin interface which allows users to write their own custom data source plugins
• An example implementation of the above interface which displays the contents of UDP datagrams as plain text (as if it were the contents of a regular text log)

[Kittyfisto]

First draft:

• A user can add a new data source almost exactly like any other data source: via the data source tree
• The clicks "Add custom data source" (a new button which shall be next to the existing buttons "Add file "and "Add folder" which presents the user with a selection, for example via context menu which data source out of the existing ones to add
• A new data source is added to the data source tree
• The user is presented with a configuration (which is entirely designed by the plugin author) which allows the user to input all information necessary to access the data source (such as endpoint)
• Once configured, the data source is accessed and display in the central area just like any other data source. The only difference being that the contents being displayed is most likely not backed by a file