Update sets management

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On the right bottom side we will display the current update set based on the opened file that is currently open in the editor.

If there is no valid update set found, we will not display update set information (like for the config file which makes no apparent sense right?)

• Onclick on this statusbar item, we have a command bound
• After clicking, display a list of other update sets other than the current one and let the user choose a different one
• After the user chooses a different update set, use the sn-edit updateset --set flag to set this new update set as the session default more info here
• Display success/error if update set was set or not and update the bottom right corner display with the newly selected update set (since that would be our new default)

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Related issue
Blocked by this #8