Test Partssplash

[jrieken]

• win @alexandrudima
• os x @sandy081
• linux @isidorn

Complexity: 3

re #53871 - On shutdown we now persist the html structure of our main parts, e.g. title part, activity part, sidebar, and status bar. This includes their dimensions and background color but no child items like buttons or labels. On next startup we show that structure to give the impression that parts of the workbench have already loaded (while we are actually still loading code and creating things).

• test that the "parts splash" works when using the native and custom title bar
• test that the "parts splash" works when opening a folder, a workspace, when opening a new, empty, window, and when opening just a file
• test that the "parts splash" works properly when hiding parts (e.g. hide statubar part)
• use workspace specific styles, e.g. have a workspace setting that defines the color of the activity part