Open the file in the msgwindow even if no line number is specified

Open the file in the msgwindow even if no line number is specified

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