Option to disable dragging/reordering of palettes

[XiaoMigros]

This PR allows disabling the ability of dragging palettes, to avoid unwanted reordering of them. It adds a checkable option to the palette context menu:

• I signed the CLA
• The title of the PR describes the problem it addresses
• Each commit's message describes its purpose and effects, and references the issue it resolves
• If changes are extensive, there is a sequence of easily reviewable commits
• The code in the PR follows the coding rules
• There are no unnecessary changes
• The code compiles and runs on my machine, preferably after each commit individually
• I created a unit test or vtest to verify the changes I made (if applicable)

[cbjeukendrup]

Looks good to me from a technical point of view. @XiaoMigros Could you please rebase it?

[avvvvve]

Looks good to me too (after confirming that it's turned on by default) BUT let's trim the copy down to just "Allow reordering palettes" please @XiaoMigros :)