Make BlendSpace toolbar need less width

[vaner-org]

Closes #119136.
This PR is deliberately small in scope for cherry-picking to 4.7, to make the experience less problematic for small screens.
A more comprehensive solution is being worked on, for 4.8, pending merge of #118970.

Shrinks width of the BlendSpace toolbar, from left-to-right, by:

• Reducing the minimum width of the Grid Step box to 2 characters, expands as needed.
• Disabling fit_to_longest_item for the Sync Mode dropdown, as it already claims variable space with "Cyclic Constant" adding a SpinBox.
• Disabling fit_to_longest_item for the Blend Mode dropdown, since everything to its right is after a spacer.
• Changing the "Open Editor" button to say just "Edit" instead, to match StateMachine tooltip "Edit state". It also doesn't need to be used as much since double-clicking was implemented.
• Reducing the minimum width of the Index box to 1 character, expands when needed.
• Reducing the minimum width of the Position boxes to 3 characters, wide enough for the minus symbol + 3 digits + decimal point, expands as needed.

Additionally changes the blend point position tooltips to not say "Value" and just say "Position" instead. It's clearer.

Before

After

cc @TokageItLab

[lyuma]

Looks good.

I was a little worried that "Edit" looks like an edit menu instead of open editor.

I would feel better if we used the same wording to describe "Open Editor" in other parts of the animation tree. But the pencil icon is still there, and everything uses the same pencil icon so that's probably good enough

[vaner-org]

I suppose "Open" was a better action word in this case. Since what's being done is more so an expansion of the node into the editor than it is an edit itself, and the blend point's name, index and position are all being "edited" right in this view anyway.

[vaner-org]

Changed to "Open" in BlendSpace1D/2D and BlendTree.
Added "Open in editor." tooltip to both to match StateMachine, added period to StateMachine's to match tooltip convention.

[lyuma]

I think Open makes sense. Seems good to me