Refactored Commands; Context menu; Text key/namespace conversion

* Refactored Commands to shorter, more concise code, writing the command names less often
* Added text key tree context menu
* Converting between text keys and namespaces (from the context menu)
Fixed renaming and deleting issues

* When confirming the Rename key dialog without changing the key, a merge dialog would open, but instead nothing should happen
* When deleting a full key but not its children, the next key would be selected, but the selection should not change
Accepting of detected text key problems

* For each type of acceptable problem, an indicator button is displayed below the causing text field
* Clicking the button toggles the accept flag
* Text keys with all accepted problems have a green icon in the tree view
* Moved all application-defined user controls to the Controls subfolder/namespace
* Refactored CultureTextViewModel, QuantifiedTextViewModel and TextKeyViewModel (regions, code restructuring, new Command structure, using ViewModelBase.CheckUpdate method)