WIP: [Feature] Setting to enable automatic background color/pattern inversion for each document

[ChrisRenfrow]

This PR aims to at least partially address #1293, specifically the dynamic dark-mode background (I'm thinking the stroke + palette inversion action might be its own feature)

Submitting early as WIP to open things up for discussion/feedback.

Here's a rough overview of where I'm at for anyone to follow along.

Initial approach:

• Replace background_pattern_invert_color_row with a multi-select option:
  • Enabled, Disabled, and Follow System
• Apply background inversion based on setting at document load or on setting change

Problem(s):

• Document settings are per-document, so this setting would have to be enabled for each document
• As someone who rarely saves what I draw (I mostly use Rnote as a one-off whiteboard/scratchpad), this didn't seem to align with my general use-case

Revised approach (we are here, presently):

• Move background_pattern_invert_color_row to general_background_invert_mode_row under the general category as a more universal setting
• Apply background inversion on any document load or setting change

Problem(s):

• The background inversion operation modifies the document locally, meaning this could potentially be a destructive/undesirable operation for any opened document if the user isn't aware of how this option may impact them

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I think what could make sense (if a little convoluted) could be some kind of hybrid between the local per-document setting and the global default where the local setting overrides the global default, but I'll keep considering more satisfactory ways to handle this.

I'm open to any direction or feedback.