keep provider selection on re-open

[koebi]

Fixes #168

[TheGreatRefrigerator]

the config.yml file is now used to store the active provider.

81d65deb79d1392c4ba3385c5e803739163d2334 sucks due to multiple reasons also documented in code and commit, which is why we probably don't want that after all:

• the defaultValue property of the Enum in the processing algorithm is not working, which is why the options have to be reordered if we want to show the currently active provider. This means indices differe between enum options and the provider list in the config, which has to be resolved when running the algorithm.
• if there are changes to the active provider between opening the algorithm window and running it (e.g. open dialog window, switching provider) it might not resolve to the correct provider.
• the selected provider can't be saved if the algorithm window is just closed and not run

our options:

• read and save active provider in processing algorithms (might resolve wrong)
• only read active provider in processing algorithms (might resolve wrong)
• don't use active provider in algorithms at all (provider has to be selected each time)

[TheGreatRefrigerator]

@koebi could you also have another look at this an give your opinion