Refactor FileSystem handling (fix absolute paths on Windows)

[BlazingTwist]

I noticed that while G works great, .G does not.
Example: "C:\abs\file.txt" :& G \ "_2"+ 0 .G (which should copy the file content into C:\abs\file.txt_2)
instead results in

io_err at .G: unable to use resource C:\tools\Aya\bin_v4_rc1\C:\abs\file.txt_2. Invalid path
   in .G .. }

So I had a look at all the uses of AyaPrefs.getWorkingDir() and updated those.

Tests

.# Windows Tests
"C:/abs/test.txt" :& G \ "_2"+ 0 .G       .# correctly copies from C:/abs/test.txt     to C:/abs/test.txt_2        preserving \r\n
"test.txt"        :& G \ "_2"+ 0 .G       .# correctly copies from <workDir>/test.txt  to <workDir>/test.txt_2 preserving \r\n
"/test.txt"       :& G \ "_2"+ 0 .G       .# behaves identical to "test.txt" on Windows (good)

.# WSL Ubuntu Tests
"C:/abs/test.txt"     :& G \ "_2"+ 0 .G   .# io_err at G: unable to use resource /mnt/c/tools/Aya/bin_v4_rc1/C:/abs/test.txt (good)
"test.txt"            :& G \ "_2"+ 0 .G   .# correctly copies from <workDir>/test.txt  to <workDir>/test.txt_2
"/mnt/c/abs/test.txt" :& G \ "_3"+ 0 .G   .# correctly copies from C:/abs/test.txt     to C:/abs/test.txt_2

:{sys.file_exists} also behaves as expected.
I did not test SaveGraphicsInstruction, Plots and File-Deletion (seems reasonable to assume that these will operate correctly)

[BlazingTwist]

Note that this is not exactly equivalent to the previous implementation, as \r\n is not trimmed to \n.
I think this is preferable when working with binary files. (Unless there's an operator for reading binary files that I missed?)

[BlazingTwist]

This change is kinda off-topic. But I figured it was reasonable enough.

[BlazingTwist]

these methods were unused