GDBGUI on windows incorrect path for loading source file

Hello,

Steps to reproduce:
1. Specify exe file and press "Load binary"
2. Press "Fetch source files"
3. Click on any source file

You will get and error "expected to find source file" in the following JS code

```
get_body() {
const states = constants.source_code_states;
switch (this.state.source_code_state) {
case states.ASSM_AND_SOURCE_CACHED: // fallthrough
case states.SOURCE_CACHED: {

let obj = FileOps.get_source_file_obj_from_cache(this.state.fullname_to_render);
if (!obj) {
console.error("expected to find source file");
```

where this.state.fullname_to_render will contains "/C:\Folder\Filename.cpp" instead of "C:\Folder\Filename.cpp"