Draft: Remove raise/lower buttons on layer/paths/channel dockable dialogues

This patch implements the first half of the UI proposal made by @prokoudine here: https://twitter.com/lgworld/status/1722004334601548000 (The floating selection discussion is still on-going, so I thought that button proposal could be handled separately)

The stated rationale is that shifting layers up and down one row at a time has been replaced by drag-and-drop, layer groups, and multi-select and is no longer needed. Removing these buttons allows the layer dialogue to be shrunk further horizontally.

I have no strong opinions, but wanted to add this merge request for discussion and potentially to implement the suggestion. One thing I notice is that this would also remove the Shift+Click button option to move layers to the top or bottom instantly (although it's still accessible in the menu). @Uzugijin, @pixelmixer, do you have any thoughts?