workspace: correct the file filters for 'open workspace...'

Bug Description:

The 'open workspace...' dialog incorrectly includes the 'all files' filter meaning that the application allows any file to be opened as a workspace which is incorrect. There is logic to reject opening the file if it is not of the correct type, but we should also adjust the filter as to not be able to choose incorrect file types.

The correct file types are the following only:

• *.theia-workspace
• *.code-workspace

Steps to Reproduce:

1. start the application (browser or electron)
2. trigger the open workspace... command
3. notice that the all files filter is set
4. attempt to open a file which is not correct (an error should appear)
5. we should fix the filter to correct this issue