Rearrange, insert & remove entries from windows

[Falki-git]

Features:

• [x] Add rearranging, inserting & removing entries to/from any window
• [x] Add renaming of windows
• [x] Add creation & deletion of new custom windows
• [x] Add locking of windows (popped out windows cannot be closed or dragged)
• [x] Add separator entry

Code:

• [x] Abstract windows in a separate class
• [x] Abstract all entries in separate classes
• [x] Add description for each entry
• [x] Add Utility static class for most common methods
• [x] Move styles to a separate static class
• [x] Persistence of every window parameter to a JSON file
• [x] Move all data updates to Update() since OnGUI() isn't optimal for readouts

Completes: https://github.com/Micrologist/MicroEngineer/issues/4

[Falki-git]

@Micrologist , this is now ready for a merge. I've updated the first comment to reflect what was implemented.

[Micrologist]

Thanks for your work! This is a much cleaner implementation than I would have produced.