Replace projects with workspace selector

[hohwille]

This story is a specialized change derived from #94:

• Remove Projects from the menu
• Remove the entire Projects screen
• Add a workspace chooser (similar like the chooser Choose your devonfw instance) to choose a workspace within the chosen devonfw instance (aka installation).
• Next to the workspace chooser there should be a + icon button with tooltip add workspace to create a new workspace. This will not open the existing Projects > New Project screen that should also be removed from the codebase but just ask for a name, create that workspace folder on submit and add it to the dropdown of the chooser. BTW: If this can be addressed with an editable combobox instead of the dropdown that does auto-completion but indicates if what I have manually typed does not match an existing workspace folder, then it gives me visual option (hover icon) to create that workspace so we would avoid an extra popup or screen for this.
• If the devonfw instance is changed, the workspace should automatically be set to main what is the default workspace. However, if you can easily preserve the state and for that instance before another workspace than main was selected that previous selection comes back this would be even better. But no special effort should be done to implement this if complicated. The real concern is that if I choose instance I1 and then switch workspace from main to ws-i1-a and then switch to instance I2 it would be a bug if the workspace still points to ws-i1-a that will most probably not exist in I2.
• If an IDE is opened, then this should happen in the selected devonfw instance AND in the selected workspace.

FYI: IMHO it would be good to first create a mock for the changed UI with the two choosers and then do a review and discussion session before implementation the actual story.