pluma-utils: Enable multiline mode on regex search

[rbuj]

Test: search ^\d{2}\.\w{3}+.* in te text below:

11 aa 222
11.aaa 222
 11 aaa 222
11.aaa 222
11
11.aaa 222

Result: found the even lines

Closes #541

[raveit65]

Looks like we can consider this as Bugfix for cherry-picking to stable branch?

[superian]

Thank you.