Selections need to act as masks for other tools; they aren't just for moving pixels

[Eben]

Selections currently have an extremely limited functionality. The first click-drag makes some selection, and the second click-drag moves that selection, even if I wanted to instead redefine my selection area without moving the first incorrect selection.

So, first things fisrt: Moving a selection should require click-dragging from inside that selection. When the cursor is within a selection (and the selection tool is active), it should change to the fleur cursor. Otherwise, it should remain as the selection tool cursor, and subsequent selections should implicitly remove the current selection (unless in cases as outlined in #3700). A single click (without a drag) should simply remove any selection without creating a new one.

Now, onto the bigger issues: The selection needs act as a mask for all other operations, not just as a means for moving pixels around. For instance, the paint bucket should fill the current selection, if there is one, and act normally otherwise. Brush tools should only put down color within the selection. Effects, such as grayscale, should only apply within the selection. Etc.

[jg]

Seems like more than a FRS change.

[Eben]

I think we need to get the "first things first" functionality in place by FRS. This change is a slight modification to the existing behavior, and so shouldn't be an invasive change like the use of selections to determine the effected area of all other tools.

[AlbertCahalan]

For the younger crowd, this is too complicated. Switching to a different tool should hide/disable the selection, if not kill it entirely.

(consider a tiny selection: now the paint tool seems to not work)