Allow area Split to be initiated in any area and give better feedback
when not allowed.

T94071: Making a Horizontal Split from the top menu bar is buggy shows an inability to start a split because the initial mouse position (when right-clicking to open the context menu) was in an area that is too small to split - even though the intention is to split the area below.

This patch fixes this problem and addresses a couple other related things.

This current init for this this operation does error checking on the area under the cursor, even though we now allow the user to start at one area and then change to another mid-operation. So these checks are removed in favor of better feedback while in the modal loop, and better error checking on apply.

The current drawing of the interactive split line only checks if the target is a global area (Top Bar, Status Bar) but not for area size. So it currently show that a split can occur on small areas which will fail. This patch makes it check for both minimum size and area type.

The current code only changes the mouse cursor between horizonal and vertical versions. This patch changes the mouse cursor to the "stop sign" if you are ever over an area that cannot be split (globals, too small, title bar, etc).

In a nutshell this patch allows this process to start in more situations and then gives much better feedback as you explore potential areas to split. The following shows feedback while moving between several areas, including one that is too narrow in the middle: