UsabILItyHub: Filter Toppings according to the Database Source

[signedav]

In the ilidata it's the category with the key http://codes.modelbaker.ch/preferredDataSource describing what db the file is made for. If there is no such category it's always displayed.

Means on import

Filters UsabILIty Hub Metaconfig Files according to the current database.

Means on topping export

It detects what db sources are used and suggests you to optimize. But you can choose none - means the topping will be displayed on every db source...

Resolves #759

Depends on opengisch/QgisModelBakerLibrary#79

[signedav]

I know this topic is a little bit special, but code review would be much appreciated @domi4484

[sjib]

Suggested change in explanation on wizard window - rephrasing:
"These are the models for which metaconfigurations / toppings are available on the Usability Hub. Select the models wanted!"

and
"Filtering by type of topping. Choose 'none' to allow all."

[signedav]

@sjib Thanks. I did not take your suggestions directly but improved mine accordingly.