Single line regex

Adds support for single-line regular expressions.

This is the "simple" version of regular expressions, where it cannot match newlines characters in the middle of the expression.  This is the behavior of most CLI tools, like `grep` or `sed`.  Notably a negative range (like `[^s]`) won't match a newline.

This is implemented (in 7d38f0512190d8f5b8e3cec6715f7ce6e50bd7e7) simply using `SCI_GETRANGEPOINTER` and feeding the regex engine line after line instead of giving it the whole buffer.

This adds a (default off) option to the Find dialog to enable multi-line regex -- so it changed the default;  but I believe the vast majority of users either will most likely be annoyed by multi-line support or not see the difference (most likely).