Alter Workspace border colour to show function state

[oceanhahn]

The active Workspace always has a 6px, thick, white-centered border around it, while Workspaces without focus have no thick border at all.
I propose that the colour of this thick border around active Workspaces be used to show different editing modes and conditions in ASEprite, with regard to which pane currently has focus (Editor Workspace, Timeline/Cel operations, Colour Picker, etc.).

• For instance, when multiple Cels would be edited, either through Frames or Layers, the border could change from white to cyan. (Related to multi-Cel transformation options mentioned in Refactor internal selection manager #1459.)
• When copy/paste functions would affect the Timeline and not Workspace, the border might be grey or orange.
• There are a few other situations I am sure, but which do not come quickly to mind.

In any case, having a quick and obvious visual cue would make it much harder to accidentally commit (and subsequently bury) incredibly unfortunate errors where multiple Frames are altered by a single, seemingly-regular selection transform, or the hated Cel-copy/paste-rather-than-pixel-copy/paste.

The change I am proposing does not alter the behaviour of any tools or functions; it just reveals what ASEprite "might do next" in an unobtrusive but noticeable way.

Possibly related to aesthetic recommendations made in #1744.

Thanks for considering it! :D

..using ASEprite 1.2.25-x64 on Win10.