Replace mainPanel in ApplicationShell

[robobenklein]

Figure out the best way to override the theiaDockPanel that's by default placed in the shell's mainPanel in the default FrontendApplication.

[robobenklein]

Points of interest during research:

• CommonFrontendContribution: contains a reference to the shell, windowService
• WorkspaceService: a little misnomer? doesn't actually appear to reference the applicationShell?
• WorkspaceFrontendContribution: contains applicationShell reference, seems hot

[robobenklein]

Found it: https://github.com/CodeRibbon/CodeRibbon-Theia/commit/91ac4c6bb47f0b4d6c63df5f368c30d1246e1790#diff-841b61763fce59f691f3ce774c3667e61b3f92d7510f6e7673586127852e4478R27-R29