Fix selecting file on navigation from menus

Used to position cursor on the first line of the list.
Extract menus/menus.c:parse_spec() function

Extracts path and line number from the spec.  Returns path and
sets *line_num to line number, otherwise NULL is returned.
Fix processing of \ in filelists menus on Windows
Add unit-tests for menus.c:parse_spec()
Fix navigation to menu items with full path on Win

Thanks to Robert Sarkozi.
Merge branch 'handle-drive-letters-in-filelist-menus'

Fix selecting file on navigation from menus with filelists.

Fix processing of backward slashes in menus with filelists on Windows.

Fixed navigation to menu items with full paths on Windows.

Thanks to Robert Sarkozi.