Add ability to have multiple sheets open in the editor

[kmwallio]

User should be able to:

• Ctrl + Click on a sheet to add it to the active open sheets.
• Sheets should appear in the editing window in the same order they appear in the library
• User should not too much of a margin or gap between sheets.

Notes:

• Only sheets in the same folder should be allowed to be open at the same time.
• Ctrl + Click on a sheet in another folder will open the sheet, but close the other sheets.