[Feature] All backgrounds available from the background menu

[letsfindaway]

Is your feature request related to a problem? Please describe.
With the flexible backgrounds as proposed in OpenBoard-org#888 we have separated the usage of background patterns into two areas:

• In the preferences, we can define the sequence of background patterns. Exactly the first six of them will then appear in the background palette.
• In the background palette, we can assign a background to the current page.

It may however be useful to be able to access all background patterns from the background palette without the necessity to first switch to the preferences.

Describe the solution you'd like
The first idea which came into my mind was to extend the background palette with a ... icon button which replaces the six patterns by a scroll pane containing all backgrounds. The user could then select from any of the existing background patterns.

Describe alternatives you've considered
When asking myself: "why are there exactly 6 patterns in the palette?" my answer is that this is just historical: with three patterns (blank, lines, squares) and a light and dark background the number 6 is logical. With the flexible background patterns however the number is no longer fixed. The idea now is to have a set of patterns that can be easily overlooked. The number 6 is not so bad, but is no longer a necessity.

Alternatives and additions to the proposal above could be:

• Always present all patterns in a scroll pane. No need for the ... button.
• Make the background palette variable width (and height?), so that the user can adjust it to keep their most used patterns in the first line.
• Use the same widget in the background palette as in the preferences and make the configuration possibilities like reordering also available in the palette.

Additional context
Depending on the chosen solution it may no longer be appropriate to present the first six patterns with a different background color in the preferences dialog. Instead we could either just remove that or adjust it to the current width of the background palette.