Unable to open source files on Windows due to forward slashes in File System browser

When attaching to a process, the source files in the file system list and the file path bar/label are prepended with forward slash for windows paths (e.g., /c:\Users\\Desktop\cpp_test\hello-world-1.cpp:1).

Steps to reproduce the behavior:
1. Create a executable using MinGW64 on Windows 10 with debug info (use `-g -O0` flags)
2. Run executable and attach gdbgui to pid of executable
3. Click on "show filesystem" button and click on any files with windows path conventions (backward slash and drive-letter)
4. Observe the file path bar/label in gdbgui (forward slash in front of windows path prevents it from opening in file viewer)
5. Manually correct file path and enter it in the textbox/dropdown to open file in viewer
6. Cannot set breakpoint in the viewer (`ERROR: No source file named c.`)

OS: Windows 10 x86-64
gdbgui version (`gdbgui -v`): 0.13.2.2
gdb version (`gdb -v`): 7.11
browser [e.g. chrome, safari]: Chrome

Reproduce the Windows 10 case with the filesystem list, file path bar/label, and file viewer using the listed MinGW64 and gdb steps. Trace how the filesystem entry becomes the viewer path and breakpoint source name; done means Windows drive-letter paths open without a prepended slash and accept breakpoints.

Written by the indexing model from the issue text.