Make isModified reflect actual modified/unmodified state of buffer

Instead of calculating the hash of the buffer every time Modified() is
called, do that every time b.isModified is updated (i.e. every time the
buffer is modified) and set b.isModified value accordingly.

This change means that the hash will be recalculated every time the user
types or deletes a character. But that is what already happens anyway,
since inserting or deleting characters triggers redrawing the display,
in particular redrawing the status line, which triggers Modified() in
order to show the up-to-date modified/unmodified status in the status
line. And with this change, we will be able to check this status
more than once during a single "handle event & redraw" cycle, while
still recalculating the hash only once.

internal/buffer/backup.go

@@ -125,7 +125,7 @@ func (b *Buffer) ApplyBackup(fsize int64) (bool, bool) {
 				if choice%3 == 0 {
 					// recover
 					b.LineArray = NewLineArray(uint64(fsize), FFAuto, backup)
-					b.isModified = true
+					b.setModified()
 					return true, true
 				} else if choice%3 == 1 {
 					// delete