Plugins may need to open a file from the message window without
specifying the line number, so allow messages of the form:

file[:line[...]]

For instance, all the following are now correctly parsed and the
corresponding file is opened:

/path/to/file
/path/to/file:line
/path/to/file: something not a line
/path/to/file:line: something
/path/to/file:line something

etc.

git-svn-id: https://geany.svn.sourceforge.net/svnroot/geany/trunk@5754 ea778897-0a13-0410-b9d1-a72fbfd435f5