Saving an existing workbook under a new name should default to Python cwd and rel. dirs should be supported

[fzumstein]

to align it with saving a new file. currently, it defaults to the Excel's default directory.

[ankostis]

Also I confirm the same problem in v0.9.x under Windows: Book.save() does not respect cwd for relative paths, and they are saved instead into e.g. %HOMEPATH%\Documents. But when reading, the same behavior is not observed.

Here it is some rough code to reproduce it:

>>> import xlwings as xw, os.path as osp

>>> cd
D:\Work

>>> fpath = r'.\foo-book.xlsx'  ## RELATIVE filepath
>>> bk = xw.Book()
>>> bk.save(fpath); bk.close()
>>> osp.isfile(fpath)
False

>>> xw.Book(fpath)
Could not connect to workbook ''.\foo-book.xlsx''
>>> osp.isfile(osp.join(r'C:\Users\user\Documents', fpath))
True


>>> fpath = r'D:\Work\foo-book.xlsx'  ## ABSOLUTE filepath
>>> bk = xw.Book()
>>> bk.save(fpath); bk.close()
>>> osp.isfile(fpath)
True
>>> xw.Book(fpath)
 <Book [foo-book.xlsx]>
'''

The same behavior applies when using UNIX-like paths.